Two seasons are considered the best for visiting Bhutan:
- Spring (March-May), with its flowers on different colours, but less clear skies
- Autumn (late-September to late-November), with coloured leaves , cooler and with clear skies
In between, from June till mid September, is the monsoon season. Winter can be cold and with snow. Altitude is a factor as well.
All the seasons have some rain, that's a good reason for Bhutan being so green.
The Bhutan tourism website http://www.tourism.gov.bt offers a wealth of information, including the event calendar