Why is there such a thing as leap year?

People generally say there are 365 days in a year. By a year, I
mean this is the time period it takes the earth to travel around
the sun: 365 days.

Actually, however, it takes the Earth 365.25 days to make this
trip. In other words, for every year we gain one-fourth of a day
and every for years we gain an extra day. If nothing was done
about this, our calendar would move backwards one full day every
four years in relation to our seasons.