Myanmar can be visited all year round. In the north of the country especially at the Inle Lake it can get quite cold during the night from November to February. Best time to visit is November to April as it is mostly dry and not too hot. May to October it is wet and humid and in the south and near the coast you can expect frequently short tropical rainfalls. In Bagan and the middle part of Burma it rarely rains even during the monsoon period but due to the heat you should allow for some time to rest after lunch and plan sight seeing and excursions in the early and later hours of the day.
The fantastic beaches at the coast of the Bay of Bengal are awaiting you. The best time to visit these pristine white sandy beaches of Ngapali and Ngwe Saung is between November to May. South-westerly monsoon is in the summer months between June and October which could bring rain and thunderstorms. There are no scheduled flights to the beach destinations during this time and most of the hotels and restaurants are closed.