The best time to visit Myanmar is during the dry season, between October and May. During this period, it rains less (or not at all, in some places) and it is not so hot. It is advisable to book holidays well in advance for this peak travel season.
The southwestern rainy season begins in mid-May and runs from July to September. The dry area between Mandalay and Pray receives minimal rain. From March to May, Yangon reaches 40 degrees. Mandalay and Bagan are often warmer.
However, except for the month of September, you should not hesitate to visit this time as Myanmar has a lot to offer throughout the year.