How do they decide which Tuesday of the month is Mardi Gras?

Mardi gras is not held on a Tuesday every month. It is a holiday that is celebrated every year, and the date is determined by the Catholic liturgical calendar. Mardi Gras is always held on the day before Ash Wednesday, which is the beginning of the Lenten season. Ash Wednesday is 46 days before Easter Sunday, so the date of Mardi Gras changes every year.