It is an all weather destination , so you can choose what fits you the most :
a) November to February : Winters are the best time to visit Alleppey and temperature stays between 17 and 32 degrees most of the time . You can enjoy houseboat ride and explore the backwaters as well.
b) March to May : Temperature goes around 37 degrees and this is not so pleasant time to visit this place,but you can surely enjoy the great deals on hotel rooms.
c) June to September : June marks the beginning of monsoons in Alleppey,though the house boat rides are not available at that time , you can sit and enjoy the natural beauty of Alleppey .