How February 29 affects calendars, birthdays, and festival dates
Introduction
A leap year occurs every four years in the Gregorian Calendar, adding a 29th day to February. Without this adjustment, the calendar would drift by about 6 hours per year — roughly 24 hours every four years — causing the Equinox and Solstice to fall earlier and earlier. After 700 years, winter would begin in June in the Northern Hemisphere. Leap years are the mechanism that prevents this drift.
The Gregorian Leap Year Rule
The rule is: a year is a leap year if divisible by 4, except for century years, which must be divisible by 400. The year 2000 was a leap year; 1900 was not. This refinement over the Julian Calendar (which added a leap day every four years without exception) reduces the Gregorian calendar's average year length to 365.2425 days, very close to the true tropical year of 365.24219 days.
Perpetual Calendar Disruption
Leap years shift the day-of-week assignment for every date after 28 February. A holiday that falls on a Wednesday in a common year may fall on a Thursday in the following leap year. This affects which day weekend observances fall on and whether a Monday holiday policy creates a three-day weekend.
February 29 Celebrations
People born on 29 February — leaplings — typically celebrate their birthday on 28 February or 1 March in common years. Legal systems differ in how they handle leap-day birthdays: some countries treat 28 February as the equivalent date for age calculations, others use 1 March. In Scotland, leaplings traditionally have the right to propose marriage to a partner on 29 February, reversing the conventional norm.
Impact on Easter
Leap years affect the calculation of Easter by shifting the ecclesiastical full moon table. The Metonic Cycle used to calculate Easter's date accounts for leap years in its 19-year pattern, but the interaction between the leap year day and the day-of-week progression means Easter can fall on slightly different dates in leap years compared to corresponding non-leap years in the cycle.
Fixed Holidays in Leap Years
Fixed holidays — Christmas on 25 December, Independence Day on 4 July — are unaffected by leap years in terms of their calendar date. However, the day of the week they fall on shifts because February has gained an extra day, displacing every subsequent date by one weekday compared to the prior year.
Other Calendars and Intercalation
The Hebrew Calendar handles Intercalation by inserting an entire leap month (Adar II) rather than a single day. The Islamic Calendar adds a single day to the last month in 11 of every 30 years. The Ethiopian calendar adds a sixth day to its 13th month every four years, following the simpler Julian pattern. Each system's approach to intercalation reflects different priorities: seasonal alignment, lunar accuracy, or astronomical precision.
Conclusion
Leap years are an elegant solution to an inescapable astronomical problem: the Earth does not take a whole number of days to orbit the sun. The extra day every four years — with the century-year exception — keeps the Gregorian calendar synchronized with the seasons and preserves the integrity of every holiday tied to seasonal timing.