Most people prefer to visit Myanmar in the cool season, probably arriving around November and heading out by the time the weather starts to turn at the end of January. Temperatures start to climb dramatically in the middle of February and April is scorching hot, peaking at around 45?C. The rains arrive in the middle of May and cool things down considerably, although this time of year can also be rather humid.
You can expect rain showers pretty much every day during the monsoon season, although in many places such as Yangon the rain tends to fall in two short showers, one in the morning and one in the afternoon. In other parts of Myanmar such as Bagan and Mandalay the rainfall is rather low.
If you are visiting Myanmar in the summer head to the hills as temperatures tend to be much lower here than in the rest of the country, meaning that you will need warm clothes if you are visiting during the cool or wet seasons.