November to February is generally considered to be the best time to visit Goa. The weather is perfect, not very cold, not very hot. It's also a good time as almost all the beach shacks are open during these months. Water sports, like banana boat ride, jet ski, parasailing, etc. are also open to tourists during these months on most of the popular beaches. Lots of music festivals including the Sunburn festival happen around December and January. If you like being in the crowd and are looking for fun party time, this is the time to go to Goa. Goa is considered to be one of the best places in India to enjoy Christmas and New Year season as the entire Goa is full of festivity.
Summer (March - June) Summers are not the ideal time to visit Goa. However, the advantages of visiting Goa in the off-season are cheaper accommodation and rates of food, booze, travel and absence of crowd and rush.
Monsoon (July - September) Goa in monsoons could be quite the dampener. However, the flip side to being here this time is that all prices take a dip. You can also enjoy the trek to Dudhsagar Falls and see it in its best form.
Winter (November - February) It is in the month of October that the monsoon starts to fade away and there is a slight chill in the atmosphere. The winter starts setting in from November. Due to proximity to the Arabian Sea, the winters are not harsh. The temperatures in winter are around 20-22 degrees Celsius.