Carnival is celebrated the week preceding Ash Wednesday, which marks the beginning of Lent. The date of Ash Wednesday, like the date of Easter, varies from year to year. In 2012 Ash Wednesday falls on February 22. Carnival takes place in the week or so before that date, although customs may vary in different destinations. Some Mexican cities like to stretch out their carnival celebrations: in Mazatlan carnival celebrations begin on February 9th which allows them to celebrate over nearly two weeks, and in Veracruz celebrations begin on February 14th. Check the official website of the carnival destination you're planning to attend for details of when and where parades and other events are being held.
More about Carnival in Mexico:
Creative Commons photo by cable369