While selecting a suitable itinerary for your visit to Bhutan, there are two other factors that will probably influence your decision: the weather conditions and the festival season.
Autumn (late September-November) is the best season for mountain panoramas, photography and trekking. Spring (March-May) is somewhat dustier and more overcast, but ideal for botanists and birdwatchers. The extraordinary diversity of Bhutanese flora is best appreciated at this time. Winter (December-February) is fine for visits to the Paro-Thimpu area of Western Bhutan, although passes leading to Central and Eastern Bhutan can be closed by snow. The days can be warm and sunny, and the light is superb, but the nights are very cold, so warm clothing will be essential for any winter visitors. Summer (June-early September) is the rainy season.
Although it can rain in Bhutan at any time of the year and you should always carry an umbrella, the monsoon deluge in summertime obscures mountains and valleys, flooding the narrow roads and sometimes causing landslides. The areas of Eastern Bhutan bordering Assam have some of the highest levels of rainfall in the world, and the countryside is infested with leeches. Nonetheless, the cultivated fields and forests are more verdant at this time than in any other season.