Our modern calendar is loosely based on the ancient Romans’ calendars. In the 8th century BCE, the Romans used the Calendar of Romulus, a 10-month calendar that kicked the year off in March (with the spring equinox) and ended in December. January and February didn’t even exist. The second king of Rome, Numa Pompilius, decided to make the calendar more accurate by syncing it up with the actual lunar year—which is about 354 days long. Numa tacked on two months—January and February—after December to account for the new days.

In around 45 B.C., Julius Caesar commissioned an expert to put aside the lunar origins of the Roman calendar and make it sun-based, like the Egyptian one. Caesar added 10 days to the calendar year and an extra day in February every four years.

The Gregorian calendar, our modern calendar, was proclaimed in 1582 by Pope Gregory XIII as a reform of the Julian calendar.

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