Mechanical Principles › Complications › Astronomical Indications

THE DAY OF THE WEEK

MENU

Semi-instantaneous mechanism for driving the day of the week

1. General Description

The display of the days of the week is one of the simplest calendar complications to achieve. Its mechanism relies on a perfectly regular and constant cycle, which fundamentally distinguishes it from Date / Calendar complications, which must account for months of unequal length. This regularity makes the day-of-the-week mechanism particularly simple to design and manufacture.

This indication is rarely found as the sole complement to the time display. It is generally added to the Date / Calendar in the simplest cases. It is also found in full calendars — often referred to as the Triple calendar — and, most of the time, in annual Date / Calendars, perpetual or secular ones. These Complications naturally incorporate the Day of the week as additional information essential for a complete reading of the calendar.

From an aesthetic standpoint, the indication of the Day of the week enriches the Dial by adding textual information to it. The simultaneous presence of both indications — Date / Calendar and Day of the week — on the same Dial marks the starting point of the large family of calendar Complications.

2. The seven Days of the week: origin and definition

The seven-Day week is a unit of Time with no direct astronomical basis. Unlike the solar Day, which corresponds to a rotation of the Earth on its Axis, the lunar Month, linked to the phases of the Moon, and the year, corresponding to a complete revolution of the Earth around the Sun, the seven-Day week is a human invention. It does not derive from any measurable celestial phenomenon.

The most commonly accepted origin of the seven-Day week is Babylonian. Babylonian astronomers observed seven celestial bodies visible to the naked eye: the sun, the moon, and the five planets known at their time — Mars, Mercury, Jupiter, Venus, and Saturn. These seven celestial bodies were associated with each of the Days of the week, according to a rotation system that persists to this Day in the nomenclature of many European languages.

In French, the names of the days of the week partially reflect this ancient heritage. Monday comes from “moon”, Tuesday from “Mars”, Wednesday from “Mercury”, Thursday from “Jupiter”, Friday from “Venus”, Saturday from “Sabbath” (of Hebrew origin), and Sunday from the Latin “dies dominica”, the Lord’s Day, of Christian origin. The seven-day week gradually became established throughout the Western world, before being universally adopted as an international calendar unit.

From a Horologist (watchmaker) perspective, the only truly important characteristic of the seven-day cycle is its perfect regularity. This cycle contains no irregularity, no exception, no jump. It passes in the same way, week after week, year after year, without ever requiring a calendar correction. This property radically distinguishes it from the Date / Calendar, which requires adjustments at the end of every short month, and brings it closer to the regularity of hours and minutes. It is precisely this regularity that simplifies the design of the corresponding Horologist (watchmaker) mechanism.

3. Mechanical principle of the day-of-the-week Indicator

From a mechanical standpoint, a day-of-the-week indicator is a simple complication. Its logic rests on a single requirement: advance the display organ by one step every 24 hours, that is, each time the hands pass midnight. The complete cycle consists of seven steps, corresponding to the seven days of the week, before returning to the first day.

The drive mechanism draws its energy from the Motion work. A branch of this Train drives a wheel called the day driving wheel, which makes exactly one revolution in 24 hours. This wheel is connected to a Finger or a Cam which, with each complete revolution, advances the day Display by one step — whether it is a Disc or a seven-pointed Star (wheel).

The day driving wheel must make one complete revolution every 24 hours. Since the movement’s Hour wheel generally rotates once every 12 hours, it is sufficient to reduce it by half to obtain the desired rate.

4. Display: disc and hand

The display of the day of the week most often takes one of the following two forms: the day disc or the star (wheel) of seven, depending on the design chosen by the manufacturer.

The day disc is a flat ring bearing the seven abbreviations or full names of the days of the week, inscribed at regular intervals around its periphery. It is positioned beneath the dial and reveals only one indication at a time, in an aperture / window cut into the dial. Its design is identical to that of the date / calendar disc, the only difference being that the day disc has only seven positions instead of thirty-one.

The seven-pointed star, or seven-branch star wheel, is used when the Display is entrusted to a Hand. This wheel, each branch of which corresponds to a Day of the week, is fixed to the Axis carrying the Hand. The Finger of the driving wheel comes to rest successively on each branch of the star, causing it to pivot by one-seventh of a turn with each Day change.

In both cases — Disc or seven-pointed star — the Display is held in position by a Neck chain (sautoir). This spring, ending in a rounded tip, engages in a notch on the Disc or between the branches of the star, ensuring precise and stable positioning of the indication between two successive changes. The Neck chain (sautoir) prevents any unwanted movement of the Display due to shocks or vibrations.

5. Display by disc with Aperture / Window

5.1 Arrangement and reading

The Day of the week Display by disc, positioned beneath the Dial in the peripheral zone of the Movement, presents each Day successively in an aperture on the Dial. This aperture is generally rectangular or sector-shaped, and positioned according to the Dial’s composition — most often between 12 o’clock and 3 o’clock, or side by side, or even symmetrically to the Date aperture when both indications coexist.

The inscriptions on the Disc can take various forms. Most often, three-letter abbreviations are used: LUN, MAR, MER, JEU, VEN, SAM, DIM in French, or MON, TUE, WED, THU, FRI, SAT, SUN in English. Bilingual watches feature two inscription tracks on the same Disc, allowing the Display language to be chosen by simply rotating the Disc by half a position using a correction Pusher. In this case, the Disc jumps two steps during normal operation.

In certain watches, the full names of the days are sometimes written out in full. This solution, aesthetically more elegant, requires a larger-diameter Disc and an Aperture wide enough to ensure text legibility. The choice of typography then takes on particular importance in the visual composition of the Dial.

5.2 Managing the seven-day cycle

One of the particularities of the day disc, compared to the date disc, is managing the return to the first day after the seventh. This transition must occur seamlessly, without requiring any intervention from the user. The drive mechanism advances the disc by one step per day, and the cycle naturally starts over after seven days, since the disc has only seven positions.

There is no complication related to the cycle of days of the week: unlike the date, which must be corrected at the end of every short month, the day-of-week indicator never requires automatic calendar intervention. It operates the same way every week, without exception. The only correction needed is manual and occurs only after the movement stops or during an initial setting.

6. Hand display

6.1 The hand on the offset dial

The display of the day of the week by hand is the classic alternative to the Aperture/Window Disc. A dedicated Hand moves across an offset (eccentric) Dial, graduated or bearing the abbreviations of the seven Days. This secondary Dial is generally circular in shape and positioned at 12 o’clock, at 6 o’clock, or laterally depending on the composition of the main Dial.

The Hand makes one full revolution of the offset Dial in seven Days, advancing by one-seventh of a revolution with each change of Day. This Display mode is particularly suited to watches providing other indications, where the calendar indications are grouped in separate secondary Dials. Legibility depends directly on the diameter of the offset Dial and the fineness of the Hand used.

The day-of-the-week Hand is driven by the seven-point Star (wheel), to which its Axis is attached. The Finger of the day-driving wheel rotates the Star (wheel) and, with it, the Hand, by one-seventh of a turn at midnight. The Neck chain (sautoir) keeps the assembly in a stable position between two midnight transitions.

6.2 The Retrograde display

Display by retrograde hand is a particularly spectacular variant of the day-of-the-week indication. Instead of performing a full circular rotation on a dial, the hand progresses linearly or in an arc from Monday to Sunday, then instantly returns to its initial position at midnight on Sunday to begin the next cycle.

The design of a Retrograde display for the day of the week is more complex than that of a circular display. The Retrograde Cam must be precisely profiled to trigger the return exactly after the seventh day. The Retrograde spring must be sized to ensure a swift and reliable return.

7. Drive mechanisms

All day-of-the-week Indicators, whether disc-type or hand-type, rely on a drive mechanism responsible for transmitting motion from the 24-hour wheel to the Display. These mechanisms fall into two main families: semi-instantaneous mechanisms and instantaneous mechanisms. This classification is identical to the one used for Date / Calendar mechanisms.

7.1 The semi-instantaneous mechanism

In the semi-instantaneous mechanism, the day change takes place in two distinct phases. During the first phase, which can last from a few seconds to a few minutes depending on the design chosen, the driving wheel gradually pushes the display element — disc or seven-pointed star — towards its next position. The display is then in slow and continuous motion, which can make reading difficult during this short transition period.

The second phase is instantaneous. When the tip of the neck chain (sautoir) crosses the peak of the tooth or of the following branch of the disc or star, it releases the accumulated energy and abruptly positions the display element in its new position. The neck chain (sautoir) then ensures the stable maintenance of this position until the next change, twenty-four hours later.

The semi-instantaneous mechanism is simple, robust and energy-efficient. It represents the most widespread solution. Its main drawback is the short transition period during which two days are partially visible in the aperture, which can create reading ambiguity in the minutes preceding midnight. This limitation is however rarely troublesome in practice, as the transition is quick and the phenomenon barely perceptible.

7.2 The instantaneous mechanism

The instantaneous mechanism guarantees a day change in a fraction of a second, exactly at midnight. To achieve this, it uses an energy-accumulating spring, progressively wound by the 24-hour wheel during the last minutes before midnight — generally over a range of ten to thirty minutes depending on the calibre. Throughout this winding phase, the display component remains stationary on the current day: energy is stored in the accumulator spring without any movement of the display yet being perceptible.

At the precise moment of midnight, a Cam releases the accumulator spring. The stored energy is instantly transmitted to the Finger of the driving wheel, which propels the seven-pointed Disc or Star (wheel) to its next position in a fraction of a second. The Day change is then imperceptible to the naked eye: the Aperture / Window or Hand directly displays the new Day without any visible intermediate transition.

This mechanism offers perfect legibility at any time. The displayed value is always clear and unambiguous, regardless of the time of day at which the watch is consulted. In return, the energy required to wind and release the accumulator spring is significantly greater than that of a semi-instantaneous mechanism. This increased energy expenditure can affect the Movement’s Autonomy and requires careful design to remain compatible with the Calibre’s running requirements. These mechanisms are mainly found in higher-end watches, where the perfection of the Display justifies the technical investment.

8. The Neck chain (sautoir): positioning component

The sautoir is an essential component of any day-of-the-week display mechanism. Its role is to hold the display — disc or star (wheel) — in a precise position between two day changes, and to participate in the drive mechanism when moving from one position to the next.

In its most common form, the sautoir is a steel spring whose end is shaped into a rounded beak. This beak engages in a notch on the day disc or between two arms of the seven-pointed star (wheel). The spring’s tension ensures that the position is maintained and gives the display sufficient resistance to external disturbances — shocks, vibrations — to guarantee the stability of the indication under all circumstances.

When changing the Day, the Finger of the driving wheel pushes the display organ toward its next position. The beak of the Neck chain (sautoir) then slides along the inclined face of the tooth or the next branch, progressively compressing the spring. When the beak reaches the top of this obstacle, it suddenly flips into the next notch: it is this flipping movement that gives the “instantaneous” character to the second phase of semi-instantaneous mechanisms. The force of the Neck chain (sautoir) is carefully calibrated by the regulator: too weak, and positioning becomes uncertain; too strong, and the driving mechanism requires excessive energy.

In fully instantaneous mechanisms, the Neck chain (sautoir) plays a different role. It does not intervene in the Winding phase of the accumulator spring and merely holds the display organ in position after the instantaneous jump caused by the release of stored energy. Its design is then optimized to provide firm retention and minimal resistance to return.

9. Regulation / Adjustment and correction of the Day of the week Indicator

Like any calendar complication, the day-of-the-week indicator is equipped with a correction system allowing the user to manually adjust the display. This correction is necessary during the initial setting of the watch, or when the watch is restarted after a prolonged period of inactivity.

The correction method varies depending on the calibre. Some movements allow direct correction via the Crown, by pulling it to an intermediate position known as the calendar’s “quick correction” position. Other calibres have a dedicated Corrector, generally integrated into the Case middle.

An important precaution should be observed when manually setting a Day of the week Indicator: the correction should never be performed when the driving mechanism is in its active phase, that is, between 10 p.m. and 1 a.m. During this window, the Finger of the driving Wheel is in contact with the teeth of the Disc or with the arm of the Star (wheel) of seven. Any manual intervention during this phase risks damaging the driving parts or the Neck chain (sautoir). This precaution, identical to that applied to the Date / Calendar mechanism, is universal and applies to all types of mechanisms for Displaying the days of the week.

In Calibres equipped with an instantaneous mechanism, the risk window is, in principle, the same, but may vary slightly in duration depending on the sizing of the accumulator spring. Some manufacturers explicitly indicate, in their technical documentation, the time range to avoid for each Calibre. In the absence of such information, the general rule of avoiding corrections between 10 p.m. and 1 a.m. remains the safest.

10. Energy considerations

The low energy required to drive the Day of the week is one of the major advantages of this Complication. The mechanism only acts once per 24-hour revolution, and each step only engages a light display element over a fraction of a Lathe. This energy efficiency makes it possible to integrate the Day of the week Indicator into virtually all types of Movement, including small-diameter Calibres or ultra-thin Movements, without significantly penalizing the Power reserve.

The situation is different for mechanisms with instantaneous drive, which require an accumulating spring and the sudden release of concentrated energy. In this case, the designer must verify that the power peak required at the moment of Release remains compatible with the capabilities of the mainspring at the end of the Power reserve. In practice, this constraint is rarely critical for the Day of the week Indicator alone. It becomes more significant when the instantaneous change of the Day is coupled with that of an instantaneous Date / Calendar and the Month Display, with all three mechanisms triggering simultaneously at midnight (once a month for the Month Indicator).