First: Find the date for the Spring Equinox, also known as the first day of Spring. It lands on March 21st or 22nd.
Next: Find the full moon which follows the Spring Equinox, and then go to the following Sunday. This will be Easter Sunday. The reason this is mentioned is because Mardi Gras, Ash Wednesday, and Easter are all tied together. Now, I'm not too sure what happens if the full moon happens to be on the same day as the Spring Equinox. Nor do I know what you do if the full moon also happens to land on a Sunday. Wow, wouldn't it be really creepy if the full moon landed on a Sunday that was also Spring Equinox?
Thirdly: Count back 47 days from Easter. If you did it correctly, you should have landed on a Tuesday. That is Mardi Gras.
Of course, it's pretty easy just to look it up on the calendar as well.
This year, it's March 4th.