The History and Rules of Leap Years
Why we add an extra day every four years, and the exceptions to the rule.
A leap year occurs when we add February 29th to the calendar — but why do we need it, and how did this tradition begin?
Why We Need Leap Years
The Earth takes 365.2422 days to orbit the Sun. If we only counted 365 days each year, our calendar would drift by about a quarter-day annually. After 100 years, we'd be off by 25 days — meaning July would eventually fall in what we call winter.
Adding an extra day every four years compensates for this drift.
Ancient Origins
The ancient Egyptians discovered the solar year was about 365¼ days long. When Julius Caesar reformed the Roman calendar in 46 BCE (the Julian calendar), he formalized this with the rule: add a day every four years.
The Modern Rules
The Gregorian calendar (1582) refined the leap year system with three rules:
1. A year divisible by 4 is a leap year 2. Exception: century years (1700, 1800, 1900) are not leap years 3. Exception to the exception: century years divisible by 400 (1600, 2000, 2400) are leap years
This means 2024 is a leap year, 1900 was not, 2000 was, and 2100 will not be.
Leap Year Superstitions
Across cultures, February 29 has attracted folklore: - In Ireland and Scotland, tradition holds that women could propose to men only on leap day - In Greece, getting married in a leap year is considered bad luck - In some European countries, a person born on February 29 is called a "leapling" and celebrates on Feb 28 or March 1 in common years
The Remaining Error
Even with the Gregorian system, our calendar gains one day every 3,236 years. No further correction has been universally adopted — a problem for civilizations 3,000 years from now to solve.