If you enjoyed the extra day today you can thank Pope Gregory XIII who reformed the Julian Calendar in 1582 mainly because it really messed with where Easter landed. That and the vernal equinox kept creeping backwards. The new calendar came to be known as the Gregorian calendar for obvious reasons. Leap years are explained by the United States Naval Observatory in this way:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.
And thus the extra day today. Which I spent a large portion of with a shovel in my hand.