Mexico is a festive nation and there are many events throughout the region that will tempt you to plan your trip specifically around them. As your trip nears, make sure to visit the tourist board of your destination city or Mexico's tourist board to find out updates of events going on while you will be visiting.
As in most of Latin America, Carnival is a great time to be in Mexico. This is a moveable feast that takes place sometime in February. Some cities celebrate this more than others, But Veracruz is the liveliest place to be in this region of Mexico.
The most solemn feast of the year is Semana Santa, Holy Week (March or April): a week or more of processions and solemnities.
In November, Mexicans celebrate
Dia de los Muertos (comparable to Halloween). This is especially marked in Patzcuaro, Oaxaca and Chiapas.
Christmas is a big event in Mexico too and a great time to visit.
All towns and cities in Mexico have their own feast day, as does every single church within those towns and cities. There is thus not a single day in the year without at least one fiesta somewhere in the ccountry