Today is the day that's inserted every four years (with some exceptions)!
"But I thought that was the 29th of February?"
Well, you'd be wrong, then; it's the 24th of February. At least traditionally (deriving from the Roman calendar—though some sources say the 25th of February, e.g. this one). See the Calendar FAQ for an explanation.
Summary: the Romans counted dates backwards from certain points. The day that was doubled was the sixth day before the kalends of March (hence the term bissextile). Since both the 24th and the 25th were counted as "sixth day before the kalends of March" in leap years, one of those is the inserted day.
Hmm... according to Wikipedia, this may be changing; they say that the European Union and the Catholic Church now use 29 February as the leap day. See its entries on Leap year, 24 February, and 29 February.