Unlike other Asian destinations Burma or Myanmar has three distinctive seasons. Cool & Dry from November to middle of March, Hot & Dry from middle of March towards the end of May and Wet & Cool from June to mid October. Overall November to March is the best time to travel throughout the country. Generally during this period the average maximum temperature should not rise above 32C-35C and it will be dry. This is when the rivers will be full of water and the country side green and fresh. Please note that Burma is a large country stretching all the way from China to the south of Thailand, there are snow-capped mountains in the north and Northwest while more mountains, hills and highlands surround the central plains. If you visiting or trekking in these areas be aware that evenings and nights can be very cool to rather cold. The coastal areas and islands will always receive refreshing sea breezes and the sea will be calm and pleasantly warm.
April & May are definitely the warmest months in Myanmar. During these months temperatures can easily reach 40C in Rangoon, the delta and the central plains. During these months the country it is still dry, and there will be less visitors in Bagan & Mandalay while the mountainous region, highlands and hill-stations will be fresh and pleasant. The coast will remain dry but with high seas higher temperatures and lots of surfing. The south-west monsoon rains start unpredictably sometime between mid May to mid June. During this period expect almost daily rainfalls and mainly in the afternoons and evenings. The central plains (dry zone) including Mandalay & Bagan will receive the least rain and will be surprisingly fresh while during June to September the Northwest coast (Rakhine state) and the delta region will receive most of the rain (200-250mm) .