There really isn’t a bad time. Remember down in Cancun you are in the tropics, it’s a true vacation paradise. Even in the coldest month (January) the average daily highs are in the 80’s, average daily lows overnight are in the high 60’s, it’s nice.
Some may argue that September and October are not good months to go because it’s “rainy season”. I personally disagree with the notion of these two months not being a good time to go. Because it’s a slower time of year, I’m always in Cancun every year in Sept and or Oct. I love that time of year. Maybe I’ve been real lucky, but I’ve had a lot of sunshine every time. My experience has been that when it does rain, it comes down super heavy. It lasts for an hour or so. Then the sun comes out and when that happens it is just surreal. The palm trunks are dark from the water, the leaves are dripping and with the back drop of the tropical sunshine it’s just gorgeous. You might also notice in the statistics below that although Sept and Oct have the most amount of rain fall, the number of days that it does rain isn’t that far off from the rest of the months. My opinion is, if I’m in Cancun and it’s going to rain, get it done quickly and give me the sunshine back. And that’s been my experience in Sept and Oct. The winds are typically calmer, the ocean is more relaxed, and the temperatures are about as good as it gets. Below are some helpful weather charts. Below the charts are some helpful service options available to you.
Average Daily High, and Average Nightly Low Temperatures by Month.