Hi Amita, You can count on it being warm, so choose your dress accordingly. As for rain, it is impossible to predict, but it tends to be a moderately rainy month. Make sure you resort or hotel has a backup plan in case of rain. We photographed a May wedding in Tulum that endured torrential downpours and the hotel was NOT prepared. At least they opened the bar (wide-open) for the guests. Good luck, Michael