Hi Simon. Bhutan is beautiful year around, it really depends on what you want to see. If you come in Spring is a good time for flower lovers, Summer is a good time to visit because there aren't many tourists around, so you can experience Bhutan without any disturbance and as it is, Autumn is wonderful because many of the big festivals take place (many tourists come this time), if you come in Winter, you will get amazing views as the sky is almost always clear, if you like to hike, this is a good time to visit, you can go on top mountains and get amazing views of some Himalayas. Dec./Jan. during the day when the sun is out, its actually pretty warm.. hot.. around maybe an average high of 17-20 degrees celsius in Thimphu, and a little warmer in lower altitudes like Punakha. When the sun goes down it gets a little cold, at night it can go down to 0 or a little below.
Deki Dorji (www.bhutandorji.com) almost 10 years ago