Kerala weather is classified into three season - Winter, Summer and Monsoon.
Winter, from October to February, are mild and pleasant.
Summer, from March to May, are hot and humid.
Monsoon, heavy rains and strong winds between June and September.
The best time to visit Kerala is during winter (cool) season, between October and February. Also this is the peak tourist season of the state.
Most of my visit to Kerala is during this season. In winter, the temperature drops a bit but the weather is never too chilly.
The weather, at this time is at its mild, to enjoy yourself on beaches. This is also a good time to see some of the more interesting colorful festivals.
Plan early and block hotels in advance - and be ready to expect crowds and to shell out premium amounts for accommodation, backwaters, nights out, and other events.
March, April and May are hot and humid (summer), though the hill stations of Thekkady and Munnar are still pleasant - a good time though for a budget holiday. The heat will probably keep you off the beaches and you may find yourself engaged in peaceful siestas in the afternoons, but expect heavy off-season discounts as far as accommodation and sightseeing, dinner and drinks are concerned.
Heavy rain falls between June and September, swallowing up most of the beaches and more than a few roads in low-lying areas, but crowds are at a minimum and hotel rates are slashed. Monsoon season is also supposed to be the best time for Ayurvedic treatments.
Kerala weather in October, November, December, January and February:
Winter season, best time to visit Kerala. Rates will be high during this season.
Kerala weather in March, April and May:
Summer season, heat will keep you away from beaches, but hill-stations will be pleasant. Best for budget holidays.
Kerala weather in June, July, August and September:
Monsoon season, heavy rains fall during this season. Best for Ayurvedic treatment.