Bissextile is a word i learned just earlier today, the extra day added to the calendar every four years-- February 29th. It's from the Latin bis sextum, meaning second sixth, which refers to some murky ancient rule in the Julian calendar that I don't really understand.
For our lifetime its good enough to say that a leap year occurs every four years, but there's the obscure exception is that there's no February 29 on century years not divisible by 400, like 1800 and 1900. Hardly anyone knows about this rule, and part of me is saddened that I will almost surely not live to apply it. I'll never get to be the smartass that points out the rule to people when everyone is tripping over themselves in 2100 thinking that it's a leap year.