In some Christian traditions, Lent is the 40-day period of fasting leading up to Easter. Carnival is the period of overindulgence before Lent, a sort of last chance to party. Traditions grew up around Carnival that are still celebrated today in different parts of the world, even where Lent is no longer observed. It's much easier to drop a deprivation than it is to drop a party. Read about how Carnival is celebrated around the world in a list I posted at mental_floss.