May is sandwiched between the pleasant spring and the wet monsoon, so exhibits characteristics of both seasons. Temperatures are generally warm (uncomfortably hot at lower elevations) but increasingly humid as the month progresses, and dusty too. Mountain views during late spring in Bhutan can be obscured by rain clouds and humidity, as the monsoon rains approach. By later in the month you'll probably need an umbrella and/or raincoat.
Altitude has a large effect on weather across the country. The southern parts of Bhutan bordering India are subtropical and can get very hot in May. Most travelers will want to avoid this area in May, as it can be uncomfortable. The town of Gelephu, near the Royal Manas National Park, for instance, sees average temperatures between 72 and 90° F (22 and 32° C) in May, and often gets even hotter.
Conditions in the mid-hills are more pleasant, with Punakha (4250 feet/1300 m.) averaging between 63 and 81° F (17 and 27° C). The capital Thimphu, which is higher at 7200 feet (2200 m.), sees cooler temperatures year round, which in May range from 55 to 73° F (13 to 23° C). Whether you find these temperatures comfortable or too warm will depend on your own preferences, but nowhere in the higher elevations of Bhutan are likely to be scorching hot in May.
Crowds & Costs
March and April are peak tourism season in Bhutan, but May is less busy as the weather isn't as perfect as in the two preceding months. You're unlikely to find any places crowded, but it's still a good idea to book tours as far in advance as you can, especially if you have preferences for accommodation.
Where to Go
If you're not short on time (or money), head into central and eastern Bhutan. These more remote areas will still largely be accessible in May, especially earlier in the month, before the rains of the monsoon make overland travel difficult, and cause flight cancellations. The central Bumthang Valley (actually a collection of valleys) is considered the cultural and religious heartland of the nation. Eastern Bhutan is far less developed than the west and may require some small-plan flights to get to, but you're unlikely to see many or any other tourists there.
If you have less time and are more interested in Bhutan's 'classic' sights, then stick to Western Bhutan. Thimphu, Paro, and the Punakha Valley/Wangdue, as well as the famous Tiger's Nest Monastery, are mainstays on most Bhutan itineraries, and for very good reasons.
What to Do
The conditions are still OK for trekking in May, although you'll need to take some waterproofs just in case, and should expect to have the mountain views obscured by cloud and humidity in places.
If you're willing to put up with that, May is a decent time to trek because camping in tents overnight at higher altitudes won't be as cold as at other times of year. Experienced and fit trekkers, as well as novices, have plenty of options in Bhutan, from the ultra-challenging Snowman Trek and the moderately challenging Jomolhari Trek to gentler day hikes through rural landscapes and communities that let you retreat to beautiful hotels overnight.
Events in May
Monk processions. On the first day of the fourth lunar month, all monks living in Punakha move to their summer residence in Thimphu (at a higher, and therefore cooler, altitude). There's a procession en route, and people line up to receive blessings from the monks.
Traveling to Bhutan in May? Check out these great itineraries
Dagala Thousand Lakes Trek - 12 Days. Enjoy views of Everest, Jomolhari, Masang Gang, and many other mountains on this six-day trek, combined with cultural sightseeing on this itinerary.
Nature and Culture in Bhutan - 10 Days. From the famous Tiger's Nest Monastery to the sleepy streets of Paro, this tour combines the simple pleasures of natural beauty and a distinctly preserved culture.
Jomolhari Round Trek and Cultural Tour - 17 Days. This itinerary combines wild adventure with Bhutan's unique culture. The trek, though challenging, is highly rewarding and one of Bhutan's best.