I visited Bhutan in June....weather was pleasant..and nice. Starting July monsoons start. You can visit Bhutan all year around...just that winters are cold.
Hi Khirabadi, October to December is the ideal time to visit Bhutan as the air is clear and fresh with sunny skies. January and February are colder, but from then until April the climate remains dry and pleasant and in late spring the famous rhododendrons bloom spectacularly, flooding the valleys with colour