Leap Days are to account for Earth completing one full revolution around the sun about every 365.25 days. Thus, we add one day to the calendar every four years in February, giving us Feb. 29.
However, Earth doesn’t complete a single trip around the sun every 365.25 days. It’s actually every 365.242181 days. Thus, adding a Leap Day actually adds slightly too much time to the calendar to be accurate.
Leap Days were first introduced in ancient Roman times in 45 B.C. with the Julian calendar. They were the first ones to add an extra day to February every four years. But over time, that slight misalignment compounded and meant the Julian calendar wasn’t aligned perfectly with Earth’s orbit.
In 1582, the Catholic Church wanted to correct this mistake by adjusting the calendar. By then, the calendar was off by 10 days from Earth’s orbit. So, on Oct. 4, 1582, Pope Gregory XIII declared that the next day would be Oct. 15, 1582 to account for that gap.
The Gregorian Calendar, as it came to be known, (and the current calendar we use today) also added the rule for Leap Days that said that we have to skip three Leap Days every 400 years. This was to account for it being 365.242181 days instead of 365.25. If a year is divisible by 100, but not by 400, we skip a Leap Day that year.
Thus, it looks like this: Skip Leap Day in 1700, 1800, 1900, but not in 2000, then go back to skipping Leap Day in 2100, 2200 and 2300.
If, when the United States was founded in 1776, we decided to abolish Leap Days, we would be exactly two months ahead (60 days) of where we should be on the calendar. That would mean today’s date would be April 29 instead of Feb. 29…
