February has 29 days in a leap year so the civil calendar stays aligned with the seasons. The Gregorian rule is a small set of exceptions, not a rumour about “every four years”.
The Gregorian rule
A year is a leap year if it is divisible by 4, except when it is divisible by 100, unless it is also divisible by 400. So 2012, 2016, 2020, 2024 and 2028 are leap years. 1900 was not. 2000 was. 2100 will not be. Our engine uses that rule for every grid from 1900 to 2100.
Check a year with the leap year checker, then open that year’s calendar — February will show 29 days when the year is a leap year.
Why the century exception exists
The tropical year is about 365.2422 days. Adding a day every four years over-corrects slightly. Skipping three century leap days out of every four (the 400-year rule) keeps the drift small enough for civil use. The Royal Museums Greenwich explain the history in plain language.
ISO weeks and leap years
A leap year that starts on Wednesday (or a common year that starts on Thursday) can have 53 ISO weeks. If you staff a 53-week rota, use a week-number calendar rather than assuming 52. See how ISO week numbers work.
Other calendars
The Chinese lunisolar calendar inserts leap months, not a 29 February. The civil Islamic calendar is a pure lunar year of 354 or 355 days and does not use the Gregorian leap rule. Those conversions are described on sources and limited on the disclaimer.