Bhutan has four seasons that behave very differently, and the choice between spring and autumn shapes what you actually see.
Autumn — October to November
This is the best window. The monsoon has cleared the air, mountain views are at their sharpest, days are dry and mild in the valleys, and the trekking passes are open. It is also the main festival season, including Thimphu Tsechu and the crane festival at Gangtey. Consequently it is the busiest and most expensive period, and hotels in Paro and Thimphu book out months in advance.
Spring — March to May
The second season. Rhododendron, magnolia and jacaranda flower across the hillsides, temperatures are pleasant and Paro Tsechu — the most accessible large festival — falls in March or April. The trade-off is haze: the air is less clear than autumn and the high peaks are often veiled. For flowers and festivals, spring; for mountains, autumn.
Summer — June to September
The monsoon. It rains heavily, particularly in the south, roads suffer landslides, trails are wet and leeches appear at lower altitudes. The mountains are essentially invisible. In exchange, the country is intensely green, wildflowers are at their peak in the high meadows, mushroom season brings matsutake to Ura, and there are very few other visitors. If your priority is atmosphere over views, it is not the mistake it sounds.
Winter — December to February
Underrated. The western valleys — Paro, Thimphu, Punakha — are cold at night but often bright and clear by day, with the crispest mountain views of the year and hardly any tourists. Punakha, at 1,200 metres, is genuinely warm. This is also crane season in Phobjikha. High passes and the northern treks are closed, and eastern Bhutan gets hard.
Festivals shift every year
Tsechu dates follow the Bhutanese lunar calendar and move annually — sometimes by weeks. If you are travelling for a specific festival, confirm the date before booking flights, not after. The major ones fill accommodation across an entire valley.
A practical warning about flights
Paro can only be flown in daylight and in decent visibility, using manual approaches. Delays and cancellations are common, particularly in the monsoon and in winter fog. Never book an onward international connection for the same day you fly out of Paro; leave a night's buffer in Bangkok, Delhi or Kathmandu. This is the single most useful piece of planning advice for Bhutan.