It turns out Leap Day is not has cut and dry as once every four years. In actuality, it occurs in every year divisible by four, 2020, 2024, 2028, etc. However, Leap Day is not observed in years ending with 00, unless that year is also divisible by four.
Why not just every four years? Because the earth revolves around the sun every 365.2422 days. If we kept to the every four year schedule, the equinoxes and seasons would eventually drift apart from the months in which they should occur.
Ancient astronomers knew a year should be greater than 365 days, but it was Julius Caesar who, in 45 BC, developed the basis for the calendar we use today. It took about sixteen centuries to figure out we had drifted ten days. To counter the spread between where we were versus where we should be in 1582, ten days were removed from October, and the current Leap Day rules were established.