Goa looks spectacular post monsoon (July onwards till around September ) as the rain brings as a fresh coat of lush green to this tropical landscape. The bonus is that's when the Goa hotels offer its deep discounts.
It's party time in Goa, that attains a feverish pitch around the New Year eve. Prices shoots through the roof. Book things (Hoteles,Flights,Trains) as they turn prohibitively expensive for last minutes bookings, that is, if something is up for booking.
The season somehow lingers on even after mid February and gradually tapers till the summer heat raises.
If beach and partying is not your idea of vacation, look for the off season periods. It's myth that Goa shuts down during summer and monsoon periods. It's not. Do different things at Goa in different seasons. Goa is a 365 days a year destination.
For Nature Vacation : Post monsoon is the best time to shoot for. Goa sits trapped between the slopes of the Western Ghats and the Arabian sea coast. Monsoon makes the rivers and estuaries that empties into the sea swell. The waters falls in the mountains turn spectacular. So is the green coat of this tropical hillside hinderland of Goa. You'll get Deep Discounts from most of the upscale resorts and hotels located along the coastline of Goa.
June-July will bring rains to Goa in the form of Monsoon season. You can enjoy the wet goa in style, though beaches would be a bit rough and dangerous to get into. Torrential rails will last till August and then its tapers till September.