I know earth can’t orbit the sun in the allotted calendar year, because that would be too flipping easy, wouldn’t it? Instead, the planet pokes along, taking 365¼ days to complete the damn journey. Ergo, a day gets added to the Gregorian calendar every fourth year (unless that year is divisible by 400 or some goofy thing).
Fair enough, but why not add the extra day to May? Or September, even? Tacking additional time to a harrowing stinker like February is nothing short of abusive. However, since the practice started in the 16th century we’re lucky it doesn’t involve human sacrifice or stonings, the really nasty stuff.
This is just one of those imponderables we can’t change, like death and taxes. So we might as well look on the bright side: if we didn’t have leap days, today would be July 15, 2017. And we’d all be a year older. Kind of freaky, isn’t it?