There are Peak and Lean seasons to visit Bhutan. Spring (March, April & May) and autumn (September, October & November) in Bhutan are considered as Peak Tourist season, whereas summer (June, July & August) and winter (December, January & February) are considered as Lean tourist season.
International tourists except Indian, Bangladeshis and Maldivians have to pay a mandatory daily fee of US $250 during Peak tourist season and US $200 during Lean tourist season to visit Bhutan.
The reason having peak and lean tourist season in Bhutan is purely based on the favorable weather and climatic conditions in Bhutan.
The moderate spring and autumn seasons with fewer chances of rain and snow in Bhutan are considered as peak season for tourist to visit and have witnessed larger portion of tourist visiting on this seasons. Summer in Bhutan brings heavy monsoon, sometime causing chaos around the country with flood, landslides and road blockages. During winter season, temperature in western and northern region of Bhutan drops below subzero making it unfavorable for visitors and for few weeks the road on higher passes remain closed due to heavy snowfall.
Spring (March, April & May) and fall (September, October & November) seasons are the best to visit Bhutan in terms of climatic and weather conditions but economically summer and winter are much cheaper to visit Bhutan due to the difference in minimum daily tariff set up by government of Bhutan for its visitors.
However Bhutan can be visited all year round.