Julian calendar is a solar calendar introduced in 46 BC. It was introduced by Julius Caesar to replace the Roman Republican calendar. It is based on the principle that a year has 365.25 days, so three years of 365 days and one leap year of 366 days. Like the Gregorian calendar applied today it has the same months of 28, 29, 30 and 31 days. The only difference with the current calendar is that there are no adjustments made three times every 400 years (withdrawal of three days over 400 years).