I do not know about Mawlamyine specifically. But Tazaungdaing corresponds to the light festival in India in November, Karttikai, celebrated widely in Tamilnadu. Rows of oil lamps are lighted on paths leading to one's house. In India it marks the end of the rains, in Myanmar, too, apparently. Monks are given robes and gifts. It is a national holiday. It is quite probable that it migrated from India to Southeast Asia.

There are some articles in google scholar about this festival. This one, for example file:///C:/Users/blue%20chameleon/Downloads/A%20Case%20Study%20for%20Development%20of%20Tradition%20and%20Culture%20of%20Myanmar%20People%20Based%20on%20some%20Myanmar%20Traditional%20Festivals%20(1).pdf