A leap year occurs nearly every four years, except at the turn of most centuries.

The longer version

Adding a day every four years overshoots slightly, because the real year is a shade under 365.25 days. The Gregorian scheme therefore skips the extra day in most century years and keeps it only when the year divides evenly by 400. The year 2000 kept its leap day; 1900 did not.

That tweak leaves a residual error of roughly one day in a few thousand years. Earlier rules that added a day without exception drifted fast enough to be noticed within a handful of centuries, pulling the equinox steadily earlier and unsettling anyone who planned the farming year by the calendar rather than by the sky. Monks computing the date of Easter were among the first to complain.

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.