Why do we have leap years and how is the extra day calculated?

We add an extra day every few years to keep our calendar in line with how the Earth moves around the Sun.

Imagine you and your friend are racing around a track that's not quite a perfect circle, it’s a little wobbly. You both start at the same time, but because of the wobble, you don’t finish exactly together every time. That’s kind of like how the Earth orbits the Sun: it takes about 365 days and a bit more to go all the way around.

Leap years are when we add that extra day, February gets an extra day, making it 29 days long instead of 28.

So why don’t we just use 365 days every year? Well, the Earth takes about 365.25 days to go around the Sun. That means after four years, that extra quarter-day adds up to a whole day. So we add one day every four years, that’s when we have a leap year.

It's like saving up loose change: after four years, you've got enough to buy a new toy, and that’s the extra day!