Myanmar has a tropical climate with three distinct seasons, although the effects of the "rainy" seasons vary 
across the country:

Rainy Season
June - September

Cool Season
October - February

Hot Season
March - May

Overall the best time to visit Myanmar is from the end of October to middle of April. Yangon is very hot and humid during the hot and rainy seasons. Although Yangon and the surrounding areas can have a lot of rain during the summer, upper Myanmar is rather dry the whole year round. The months of July through September can be very pleasant in Bagan and Mandalay as the air is clearer with less dust and only light rainfall. The coastal areas should be avoided from May to the end of September. In the Shan Hill and Mrauk U the temperatures at night during the winter months can be surprisingly chilly ranging from O°C to 8°C.