Myanmar calendar is based on Lunar Cycle. Usually, it has 12 months in a year but to adjust with sidereal years, it has 13 months every 2-3 years. If you calculate with Western calendar, there will be slight different. Myanmar people do not have problem with living on Myanmar calendar and on Western calendar at the same time.
Myanmar is a land of festivals. It has traditional Burmese festivals, ethnic festivals, religious festivals and special events. The pagoda festivals are usually celebrated at famous pagodas and they are usually held around full moon days during January, February, March and April. Moreover, there are people who believe in Nat (Spirits) and so, there are Nat festivals too.