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The Julian Calendar
During the Roman republic, the year was only 355 day long and the missing days were added every other year between February 23 and February 24 to form with the last five days in February the mensis intercalis (intercalary month). This operation was entrusted to the pontiffs, who were not always keen on doing it. As a result, the official year was often out of tune with the seasons and the harvest festival would sometimes be celebrated in the middle of winter!
To put some order in this mess, Julius Caesar began by adding 90 days to the year 46 BC (which lasted for 445 days and was called the year of confusion). Then, he created the current cycle of 4 years with 3 years of 365 days and one leap year of 366 days. The extra day was obtained by doubling February 24, sixth day before the calends of March. The extra day was called ante diem bis-sextum kalendas Martias, which is why the leap year is also called bissextile.