Talking about the best time of year to visit Mexico is like trying to explain the best time of year to visit the United States or Europe. Mexico is a big country and the answer to "What is the best time of year?" is "It depends".

 In general, there is more rain in Mexico during the late summer months: August and September. This also coincides with hurricane season on the east and west coasts. To guarantee a sunny day, you probably don't want to go to Cancun or Puerto Vallarta in late August. But you can get some of the best travel deals during those times and Tulum is great in early September if you like empty beaches and a dramatic-looking sky. Then again, there are probably a lot of people who probably wish they weren't in Cancun last October during Hurricane Wilma. 

Central Mexico is on a high plain, which means that even in the summertime, the nights can be cool in places like Mexico City and San Miguel de Allende. Southern coastal cities like Puerto Angel, Mazatlan or Tulum are going to be warm most of the year, and in the summer, quite hot. If the heat bothers you, be sure to reserve a hotel room with air conditioning! Baja California is a hot desert that just happens to be next to the ocean. Valle de Bravo outside of Mexico city is a lake in the mountains with typical mountain weather...warm during the day and cool at night.

Unless you plan to climb one of its many volcanoes, there is one thing you will probably never have to dress for on your trip to Mexico: snow! 

Here is the link to Mexico : Major Cities Weather Forecast. Click here for a look at last year's hurricane and tropical storm information, as a predictor for the best times to go (or stay home) this year.