November through February is peak season for good reason: the air is clear, the temperatures are pleasant (cool enough for a sweater at night), and the mountain trails are dry. This is when the sky lives up to the town's name.
March to May brings the burning season — farmers clear fields across the north and the valley fills with smoke. Visibility drops, the air is harsh, and outdoor activities lose their appeal. Avoid this period if you can.
June to October is monsoon season. The mountain turns intensely green and often wreathed in cloud. Rain can close some trails and the summit trek becomes difficult. That said, the valley is beautiful in the wet season and visitor numbers drop substantially — if you don't mind the mud and the occasional day stuck indoors, you'll have the place largely to yourself.