BirthdayEarth doesn't orbit the sun in a neat 365 days - it takes about 365.2422 days. Without correction, our calendar would slowly drift out of sync with the seasons. Leap years exist to fix that drift.
The rule
A year is a leap year if it's divisible by 4. There's one exception: century years (like 1900 or 2000) must also be divisible by 400 to count as a leap year. That's why 2000 was a leap year, but 1900 was not.
Why this matters for birthdays
If you were born on February 29, your birthday only exists on the calendar once every four years. Most people in this situation celebrate on either February 28 or March 1 in non-leap years - our Days Until Birthday calculator handles this automatically.