The best time to visit Xiahe is between May and October, when the grasslands around town are green, the days are long, and the Sangke nomads are present with their herds. July and August are warmest but also bring the monsoon fringe — expect afternoon rain. September is arguably the best month: the crowds thin, the light is excellent, and the air has a clarity that the summer haze obscures.
Winter is genuinely cold — temperatures drop well below freezing at night and snow is common — but the monastery is serene and the pilgrim traffic continues regardless of the weather. The Monlam Festival, held in the first lunar month (usually February or March), is the biggest annual event, drawing monks and pilgrims from across the Tibetan plateau for a week of ceremonies, debates, and the unfurling of a giant thangka on the hillside opposite the monastery. If you can time your visit around it, do.
The town occasionally closes to foreign visitors during politically sensitive periods — around major anniversaries or after incidents elsewhere in the Tibetan regions. Check conditions before you go.