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1690s, from French, literally "fat Tuesday," from mardi "Tuesday" (12c., from Latin Martis diem "day of the planet Mars;" see Tuesday) + gras "fat," from Latin crassus, "thick." Day of eating and merrymaking before the fasting season of Lent.
An annual festival held in France on the day before Ash Wednesday, the first day of Lent. Mardi Gras is French for “Fat Tuesday” — meaning it is the last opportunity to eat rich food before the fast of Lent begins. It is related to celebrations elsewhere, called “carnivals,” from the Latin words carne and vale, “meat” and “farewell,” meaning a farewell to meat before the abstinence of Lent.
Note: New Orleans, Louisiana, is famous for its Mardi Gras celebration, as is Rio de Janeiro, Brazil.