Welcome to the not exactly every four year calendar oddity known as leap day! Just follow these three simple rules:
1 – Add an extra day every four years
2 – Except for years that are divisible by 100
3 – Unless that year is also divisible by 400, in which case you do add the extra day
Turns out that February 29th, 2000 was a special type of leap day that won’t happen again until February 29th, 2400.
Here is the visual version:
For the etymologists in the room check out the word bissextus.