It takes our big blue and green home we call Earth approximately 365.25 days to complete one elliptical orbit around the Sun. So that our natural seasons don’t get slung out of whack with our constructed calendar, every four years there is an extra day added to February. We call this day February 29th (boring) but we call the year Leap Year.
If you are sitting around in any given year, wondering if it’s a Leap Year, all you have to do is divide that year by 4. We have a February 29th this year because 2020 is divisible by 4. How do we know a number is divisible by 4? Look at the last two digits and see if they are divisible by 4. 2036 will be a Leap Year because 36/4 is 9. Divisibility by 4 is not quite as fun as testing for divisibility by 3, and not nearly as quick as checking for divisibility by 5, but it’s not as ridiculous as testing for divisibility by 7.
This would all work out well if it took the Earth 365.25 days to orbit the Sun. But it takes a bit less than that (approximately 0.01). So, if a year is divisible by 100 (such as 1900), then it is NOT a Leap Year. There is yet one last exception. If a year is divisible by 400 (such as 2000), then it IS a Leap Year. Apparently, this makes everything work out pretty nicely. At least the seasons won’t be out of whack in my lifetime.￼
My parents used to throw a big Leap Year party. 1968, 1972, 1976, … As my mom figured it, they would only have to open up their NYC apartment once every four years to the thirsty masses. She is one smart cookie.