Kandy's climate is milder than the coast — higher altitude means cooler temperatures and more frequent cloud cover year-round. There are effectively two monsoon periods: the southwest monsoon (May to September) brings rain from the west, while the northeast monsoon (October to January) affects the eastern slopes. Kandy sits in the hill country transition zone and gets some rain from both monsoons, though it is rarely completely wet for extended periods.
The Esala Perahera festival, held in July or August (the date is set by the Buddhist lunar calendar), is Kandy's most spectacular event: a ten-day procession of elephants, dancers, drummers, and torch-bearers that culminates in a nighttime parade of the tooth relic through the city streets. Accommodation books out months in advance; prices triple during peak perahera nights. It is worth planning specifically around if you can.
The dry season from January to April is generally the best time to visit for reliable weather. December is pleasant but busy with Sri Lankan and foreign tourists.