The best time to visit Bhutan is in Spring and Autumn, when the weather is mild and all the major festivals take place. Spring lasts from March to May and Autumn lasts for September to November in Bhutan. Autumn is even more preferable as it comes with clear blue skies and gorgeous views of high Himalayan Mountains. During this period, you can capture some of the most picturesque moments from the passes and other vantage points and you can count on an unmatched traveling and trekking experience. If you are a bird lover, you can spot some rare black-necked cranes that migrate to central and eastern Bhutan in the Autumn season, in preparation for the coming winters. In Spring, skies are not so clear but the beautiful blooming flowers make for some of the most beautiful views of valleys and mountain slopes covered with blooms presenting a riot of colors.
The only downside of visiting Bhutan during the busiest times is that there are a large number of foreign travelers and tourists visiting the country at this time and you need to make Druk Air reservations well in advance to ensure that seats are available and grab the best seats for yourself. You are advised to avoid the monsoon season during the months of July and August as it often comes with heavy downpour and occasionally with flooding and landslides. Many travelers these days are looking forward to the advantages of off-season travel to Bhutan as well. They like the idea of lesser number of tourists in Bhutan and reservations are much easier to get too. However, in the summer and monsoon season ranging from June to early September, you may encounter rain while in winter months lasting from December to February, it can be quite cold in Bhutan. It is also possible that some of the treks are closed during these off-peak seasons due to the bad weather conditions. But if you are there just to see Bhutan, you will find it spectacular and mesmerizing at all times of the year.