How is a date chosen for when Mardi Gras will be?

The date of Mardi Gras is determined by the Christian liturgical calendar. It is always the Tuesday before Ash Wednesday, which is the beginning of the Lenten season. The date of Ash Wednesday is determined by the date of Easter, which is determined by the lunar calendar. So the date of Mardi Gras can vary from year to year, but it is always in the month of February or March.