The best time to visit Bhutan is Spring (March, April and May) and Autumn (September, October and November), when skies are clear and the high mountain peaks are visible. This is the ideal time for trekking and for traveling throughout the country. You’re likely to get wet no matter what the season, but avoid the monsoon, June-August. In the central belt, including Paro and Thimpu, expect sunny skies and temperatures ranging between daytime highs of 50′ F and nighttime lows of 32’F from November to April. Late spring and fall have average temperatures ranging between 60′ F and 30′ F. In summer daytime highs will reach about 70’F, and there will be light periods of rain. The best times to visit the high mountains of the north are in late spring, fall, and early summer. Temperatures will range between highs in the 50s or 60s F, and nighttime lows in the 40s. Winters are below freezing.
Having said that, it is cheaper (US $200 per day per person) to travel Bhutan in Winter and Summer. Winter (December, January and February) may be cold for trekking in Bhutan but since it is warm and sunny during the day in the valleys it is favorable to do cultural tours. You will not get to experience big cultural festivals in winter and summer but it is in the winter that most homes in rural Bhutan perform their annual ritual and this would be a great travel experience. The people in rural Bhutan are very hospitable and they will be very happy if you drop by as a guest as one of the main purpose is to entertain guests and they cook for hundreds of people on that day. It is cheaper to travel
Summer (June, July and August) is not ideal to travel Bhutan because of the rain. But do note that it does not rain heavily and when it does, it only last for a few hours and only rarely it will rain throughout the day.