Leap years add an extra day in February to keep calendars aligned with Earth's orbit.

The longer version

A year of 365 days falls short of the true orbital period by nearly a quarter of a day. The gap is small but it accumulates: four years later the calendar sits about a day behind the sky, and after a few centuries the labelled dates no longer match the weather they once described.

Slipping one extra day into the calendar every few years absorbs that leftover time. February received it because it was already the shortest month and sat at the end of the old Roman year, so lengthening it disturbed the fewest other arrangements. Without the correction, midsummer holidays would slowly slide into genuinely cold weather over the centuries.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.