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Photo: Infrogmation of New Orleans, CC BY-SA 4.0 via Wikimedia Commons
The Tuesday before Ash Wednesday and Lent, and many towns hold a carnival. (Literally, say goodbye to meat, carne vale.) It's usually in late February but varies with the date of Easter, which follows a part-lunar calendar.
Mardi Gras, also known as Shrove Tuesday, is the final day of Carnival ; it thus falls on the day before the beginning of Lent on Ash Wednesday. Mardi Gras is French for "Fat Tuesday", referring to it being the last day of consuming rich, fatty foods, most notably red meat, in preparation for the Christian fasting season of Lent, during which such foods are avoided.