Bhutan has two peak seasons and they are peak for good reason. Spring (March to May) brings warm days, clear skies, and entire hillsides of rhododendron in bloom -- pink, red, and white. The Paro Tsechu festival falls in spring and is one of the country's most spectacular events. Autumn (September to November) offers crisp air, brilliant blue skies, and the best mountain views of the year. The Thimphu Tsechu happens in early autumn. Both seasons are ideal for trekking.
Summer (June to August) is monsoon season. Roads flood and landslides block highways, sometimes for days. The southern lowlands become oppressively hot and humid. That said, the high-altitude meadows are carpeted with wildflowers, including Bhutan's national flower, the blue poppy. If you can handle the rain and the occasional road closure, you will have the place almost to yourself and pay lower prices.
Winter (December to February) is cold, especially at altitude. Passes above 3,000 metres may be snowed in, and some eastern routes become impassable. But the western valleys around Paro and Thimphu remain accessible, festival-goers attend the dramatic Punakha Drubchen and Trongsa Tsechu, and visitor numbers drop sharply. If you dress warmly and stick to the western circuit, winter can be a rewarding time to visit.
Temperatures vary enormously with altitude. Thimphu at 2,300 metres sees daytime highs of 25 degrees Celsius in summer and lows around minus 5 in winter. The southern border town of Phuntsholing can hit 35 degrees in summer. Always bring layers, even in the warm months -- evenings cool down fast in the valleys.