When we were there in July it was a 60% chance of rain every day (we went for a week). It rained for a SOLID TEN MINUTES on one single day. That was it LOL
If the weather forecast were true, Cancun would be one of the wettest places on Earth. It isn't! We got back from the UK on Aug 10th and the number of 'horrible' days since then has been about two. The number of days when it rained at some point about another 8. That leaves about 40 days when it has been great weather, all day long.