Today date is 29th Feb adding one day to the month of February, making our year 366 days long instead of the common 365 days due to leap year.
The leap year comes after every four years February 29 in our calendar. The theory behind the leap year is that it comes down to the fact that takes the earth slightly longer than 365 days to rotate around. Indeed it takes a little less than a quarter of a day to make the round. Therefore, if we kept our calendar steady, our seasons would creep, eventually leading our seasons to shift.
To counter this shift, we add a leap day every four years. However, that is just a little too much and our seasons would creep the other way. The way we handle this is to skip century years that our divisible by 400.
Leap years are very important because it play a key role in keeping seasons and our calendars working properly.