When earth makes Full Orbit of moon is normally considered as the poya day in Sri Lanka every month.
[1][2] Uposatha is important to Buddhists all around the world, who have adopted the lunar calendar for their religious observances.
Owing to the moon's fullness of size as well as its effulgence, the full moon day is treated as the most auspicious of the four lunar phases occurring once every lunar month (29.5 days) and thus marked by a holiday.
[3] Every full moon day is known as a Poya in the Sinhala language; this is when a practicing Sri Lankan Buddhist visits a temple for religious observances.
[3] Generally shops and businesses are closed on Poya days, and the sale of alcohol and meat is forbidden.