Cancun is essentially almost always warm.  The "coldest" time of the year in Cancun is December and January where the average daily highs are 83 degrees, 23 celsius.  Warmest time is July and August with an average daily high of 92 degrees, 33 celsius.  I put two charts below to show the average daily high and average overnight low for each month.  Below this chart I included another one for average rainfall by month.