Cancun is warm all year round, with very small changes in temperature from month to month, so it really doees not matter when you are planning your vacation, weather wise. Over the year the average temperature will change from 80 F (26 C) to 90 F (32 C). The warmest month tends to be July, however constant sea breezes will help to limit the summer heat, the average wind speed in Cancun is about 9 mph independent of the month.
The coldest month tends to be January, however the beaches and Ocean are still warm enough for swimming all year round. Hurricane and Tropical storm season are typically from August to October, however over the course of a year in Cancun it will only rain approximately 60 days out of a year.