The most popular time to visit Bhutan tend to be the spring months of March, April, and May and the autumn months of September, October, and November because the weather is more mild and many of the significent festivals take place during these months.
In the autumn the skies are clearer and blue; affording views of the high Himalaya and various vantage points. Autumn is also the time of year when the black-necked cranes migrate to central and eastern Bhutan for the winter.
The skies are less clear during the spring and the flowers are in bloom. The downside of traveling during these peak months is that there are more tourists in general, and Druk Air reservations need to be made well in advance to ensure availability.
The summer months of June, July, August, and even early September are typically the monsoon months, so you may encounter rain fall.
The winter months of December, January, and February can be cold, but the skies are clearer and blue. Some tours & treks can be closed during these months due to weather. The upside of traveling during the off-peak months is that reservations are easier to secure, and there will be fewer tourists around. But truly, Bhutan is spectacular any time of the year, and your trip will be an wonderful adventure no matter when you decide to visit!