We are so lucky this year. We get an extra day! Most years are made up of 365 days. This year we get 366!
It takes the Earth one year to go around the sun once. A year is 365 days. Yet, the truth is that it takes our Earth 365 and one-fourth days to go all the way around the sun. What do we do with those leftover one-fourth days? Why, we save them until they make one whole day, that's what we do! We have three years of 365 days, followed by a leap year of 366 days. We have a leap year once every four years. It keeps our seasons in order, so that winter doesn't drift into our spring months. (It would take a very, very long time to do that.)
You would expect that extra day to be added at the end of the year, right? Wrong! It's added to the end of February, the shortest month of the year. Why February, you ask? At one time February was the last month on the Roman calendar. The Romans did just what we would do; they added the leap day to the end of their year. Now, even though February is the second month on our calendar, we still include leap day as February 29.