On February 29, way back in the year 46 B.C., the first leap day happened!

This extra day only happens once every four years to compensate for the fact that the solar year is one-quarter of a day longer than the calendar year. But the first leap year actually had an extra 90 days, not just one. Before Julius Caesar adopted this new calendar, the Romans were using a 355-day lunar calendar. They added an entire leap month every once in a while when the seasons became too out of sync with the calendar, but only when Roman priests arbitrarily decided it was time to do so. To make things even more confusing, Caesar made the year 46 B.C. 445 days long to reset the calendar. The new Julian calendar became effective the next year and finally put an end to all this calendrical craziness.