One Extra Day Every Four Years

A leap year adds February 29 so the calendar year stays in sync with the solar year. The rule that decides it has exactly three clauses: the year must be divisible by 4; if it is also divisible by 100 it must additionally be divisible by 400. That triple test is what separates 2024 (leap), 1900 (common), and 2000 (leap).

The Drift the Rule Fixes

The Earth's orbit takes 365.2422 days. A 365-day calendar loses almost a quarter-day each year, so seasons would drift a full month every 120 years — Christmas would eventually fall in summer. Adding a leap day every four years (365.25) overshoots slightly, and the century exceptions fine-tune the average to 365.2425 days, accurate to about one day in 3,300 years.

Leap-Day Trivia

Someone born on February 29 celebrates their "real" birthday only in leap years — about 5 million people worldwide are leap-day babies, roughly a 1-in-1,461 chance for any birth date. Contracts and software that handle dates have special rules for the day, which is why date libraries treat February 29 so carefully.