Hi Suzie&Karl.
I usually go around beginning of April/May. I've been to Moon Palace Golf resort, and Sandos Caracol in Riviera Maya (this one, I wouldn't recommend)
The Moon Palace is the most amazing place we have ever been too, and that is where we are going for our honeymoon, which will be May.
Here's a website with all the averages, sun, rain and tempretures etc. I usually avoid months from Aug - Nov as this can be Hurricane season. April & May are ment to be the best months, and boy are they hot then! Also as rain goes, it litterally is short, sharp bursts. We got caught in it one day the rain poured from the sky for around 2 minutes, and we were drenched, 5 mins later we were bone drying and roasting in the sun again!
Hope this helps... I'm a huge fan of Mexico as you may be able to tell. The journey isn't a problem, only when you come home, coz I'm usually depressed. LOL
http://www.holiday-weather.com/cancun/averages/