Okay kiddo, so you know how we have a calendar with all the months and days, right? Well, a really long time ago, the Romans decided they needed a calendar too.
Their calendar had 12 months too, just like ours, but each month had a different number of days. Some months had 31 days, some had 30, and one month only had 28 days. They gave each month a special name too, like January, February, and so on.
But there was a problem. The months didn't match up exactly with the seasons, like when it was supposed to be spring or winter. So the Romans decided to add an extra day to some of the months, to make up for the extra time in the year.
They also created a special day called leap day, that they added to February every 4 years. That way, the calendar would stay in line with the movement of the Earth around the sun.
Nowadays, we use a modified version of the Roman calendar, but we still have 12 months and we still have leap day. Isn't that cool?