Weather in Cancun is warm all along the year. On coldest months, a light jacket will be more than enough to feel yourself comfortable, as lowest temperature usually is 67°F, on January.
Rainy days are really few in Cancun, mostly on May-June or October-December, but anyway they do not last more than half of a day and then the sun is shining again.
Unless there is an upcoming storm or tropical depression (very unlikely too), you should not worry about the weather on your trip.
If you have the chance to see the moon rise when the sky is already dark, you will be surprised how spectacular it is. In Cancun, the moon rises from the ocean side, and its light draws a beautiful pattern over the ocean.
Cancun weather reports: http://www.holiday-weather.com/cancun/
UV levels in Cancun
Due to its geographical location, Cancun has very high UV levels all along the year. People that are not in use to take care about sun, because they do not have high levels of UV at their cities, usually get burn in Cancun. When talking about tanning in Cancun, we always recommend to take it easy, as Cancun’s sun can be very abrasive (rarely below 6 UVI).