Most of us know about the hurrican season in the Caribbean Islands, but when is the best time to travel to Mexico which isn't necessarily subject to the same issues?
Most people enjoy Mexico the most between late September through late April(Don't forget that the February through March timeframe is the favorite of Spring Breakers). Inland Mexico does have cold weather as well, which is something to consider if you travel in the winter months. You may want to avoid the resorts in August since that is a very popular time for European tourists to travel and the resorts a much busier.
Think about the climate you enjoy the most before you choose your destination and your timeframe when you plan your Mexico vacation. The coast is hot and humid and perfect for beach goers while the higher elevations inland are more arid.