During the winter months, Cancún's average high temperatures are in the mid 80's F (29 C) with lows in the upper 60's F (19-20 C), so evenings can be a bit cool. Summer months are a bit hotter, with highs around 90 F (32 C) and lows in the mid 70's F (24 C).
Being somewhat tropical, the Cancún area gets a bit of rain year round, with the September and October being the rainiest months as well as the most likely months for hurricanes and storms.
However, rains are usually afternoon or nighttime showers. Even if it does rain, Cancún does have many nice indoor malls to keep you busy for a while.
Cancún weather requires that you bring light ponchos for rain gear. These can be purchased from most camping stores. Many stadiums also carry a light poncho to sell to fans of local football teams. These work great as well. They are small and light and easy to carry anywhere you go as a rain cloud can come in no time and dump a monsoon on your head