You can visit now, as it's peak season of Bhutan as temperature do not go very low, almost all mountain you will see snow covered.
I visited Bhutan in December end and I enjoyed everything there specially snowfall, the only problem is during December end is temperature. it go upto -10 or more I survived -12 temperature during my trip.
The best time to visit Bhutan is during the spring months of March to May when the valleys come alive with flowers in bloom. The Himalayan peaks are at their most visible with the clear skies of October and this is the best time for trekking though temperatures will be on the cold side. June to August is considered the low season due to the rains, which makes trekking less of an option.