Kollam is pleasant from October to February, when the monsoon has finished and the air is cool and clear. March through May gets hot and humid. The Southwest Monsoon arrives in late May or early June and lasts through September — the backwaters are beautiful in the rain but boat services can be disrupted.
The Kollam Pooram temple festival at Asramam Maidan usually takes place in April, with processions of decorated elephants, drummers, and fireworks. It is one of the most spectacular temple festivals in southern Kerala and worth timing a visit around.