Why is there a leap day?
The Earth does not orbit the sun in the exact amount of time it takes the Earth to complete 365 rotations on its axis. That means a solar year, or the time it takes the planet to complete an orbit around the sun, is slightly more than our calendar year. It takes 365.242 days to get around the star.
The 365-day calendar does not account for that extra quarter day. So, once every four years, an extra day is added to the calendar. That realigns the calendar created by humans with the solar year.
A video from James O'Donoghue of the Japanese space agency (JAXA) shows what this looks like. It's not perfectly to scale, but the animation quickly demonstrates why that extra day once every four years keeps our calendars synced up with the planet's orbit.
If we didn't observe leap years, the seasons would drift through the calendar at the rate of about one day every four years. We're talking about large frames of time, but eventually, December would take place during the summer without those leap days. Adding in an occasional extra day aligns our calendar with the astronomical seasons.