The dry season (November through April) is the time for trekking and village visits — clear skies, cool mountain nights, and trails that are not slick with mud. December and January can be cold at altitude; bring a fleece.
The wet season (May to October) brings reliable afternoon rain and the risk of typhoons. The rafting season runs from about June through January, when the river is high enough for the best rapids — so for whitewater you have to accept some rain.
Ullalim Festival, the provincial cultural celebration, is held in mid-February: indigenous dance, the gangsa gong ensembles, and food. Bodong Festival at the same time of year is the formal commemoration of the peace-pact tradition. Both are worth timing a visit around.