Myanmar has three-season(tropical climate)cycle that follows the classic : dry and wet monsoon climate' pattern common to other parts of mainland Southeast Asia.
The raining season from mid-May to mid-October;the cool season(cold season)from mid-October to mid-February to mid-May before the rains begin.
The best time to visit Myanmar is from mid-October to mid-May during open season.
Monsoon starts between mid- May and mid-June to late October.
Generally says , traveling in the rainy season is not particularly difficult, though unpaved roads may occasionally be impassable.
Dry period begins from November to May with lower relative temperature. During December and January the temperature drop to near freezing at night in the highlands area such as Shan State, Kachin State and Chin State(Naga Land)
The temperature start to rise during March, April and May. The annual water festival , when people douse each other with cold water , takes place in April at the height of the hot season.
You'll find a nifty link to CNN.com OR Myanmar Meteorology and Hydrology Department