Leap days are extra days added to the calendar to help synchronize it with Earth’s orbit around the sun and the actual passing of the seasons. Why do we need them? Blame Earth’s orbit around the sun, which takes approximately 365.25 days. It’s that .25 that creates the need for a leap year every four years. This is why we have a February 29th in 2020.
Fun fact; less than 0.07% of the world’s population was born on a leap day.