Leap Day 
Thirty days has September, April, June, and November. All the rest.... well, you know the rest — and if you don't, that's what the Internet is for. But this little jingle reminds us how neat and orderly the modern calendar really is. Each month has a set number of days, and each year has a set number of months.
There's just one little blip — a blip that appears today: Leap Day. It's added to every fourth year to keep the calendar aligned with the seasons.
Earth's annual march around the Sun takes 365 days and almost six hours, and those extra hours have to be accounted for to keep the calendar in sync with the seasons.
The basic calendar we use today was instituted by Julius Caesar — a system of 12 months, each with a fixed length, for a total of 365 days. Even in Caesar's time, though, those six extra hours were well known. So an extra day was added to every fourth February, making the years average out to exactly 365-and-a-quarter days.
But the true year is actually a few minutes less than that. By the 16th century, those minutes had added up to a big difference. So Pope Gregory the 13th ordered a change in the calendar: Leap Day is dropped from three of every four "century" years — years that end with double zeroes.
Even this system isn't perfect — it leaves an error of one day every 3300 years. Still, we've got quite a few centuries to go before anyone has to tinker yet again with the rules for Leap Day.
Script by Damond Benningfield, Copyright 2011