Best time to visit Kerala depends on what you're looking to do as the region has three specific seasons. December, January & February are cooler and drier, yet busier, months; March, April & May is the hot and humid season; and the rest of the year is the southwest and northeast monsoon season.
"February and March are wonderful, because places and people are more relaxed after the Christmas rush. October and November are also great, as it is fresh off the monsoon and everything is luscious and green. For hiking March is perfect with clear blue skies - the sort of weather where you wake up with your eyes closed and still know it is a sunny day."
~Local Villager"The Easter holidays are good for families and August/September is still low season in a lot of the lowlands, but you need to book flights well in advance for that time of year. Also we used to warn people about the second monsoon, in October/November, but it has virtually fizzled out."
~Travel guide
"Activity wise, June and July is not advisable, because of the monsoon, especially trekking. It is fine for sightseeing as it doesn’t rain all day, and I love driving during the monsoon as everything is so green and the waterfalls are very full. Also, if you want to enjoy festivals, come in March and April, our summer period. There is a lot of tropical fruit during that time too, so we have mango festivals and so on. "
~Local Guide