Carnival refers to the season, which begins on the 12th night after Christmas (Jan. 6) and ends the day before Ash Wednesday.
Mardi Gras, which is French for Fat Tuesday, always falls 46 days before Easter. (Some use a formula that says it is the day before Lent, which begins 40 days before Easter, excluding Sundays.)
The Roman Catholic Church uses a formula to determine the date of Easter. It falls on the first Sunday after the Paschal full moon, which does not necessarily coincide with an astronomical full moon.
If the Paschal full moon falls on a Sunday, Easter is the following Sunday. The earliest Easter is March 23 and the latest is April 25.
(Do not attempt to work this formula on a calculator. Remainders come out as a decimal number. The formula is old and requires old-time long division with pencil and paper and a whole number for the remainder.)