Poya Day is not a single day but a name for any holiday in Sri Lanka that is held to mark a full moon.
There are around a dozen Poya Days every year, in conformity with the Buddhist lunar calendar and moon-phase marking system.
As 70 percent of Sri Lankan are Buddhist, most of the population celebrates each Poya Day as of religious significance. In Sinhalese, "poya" is derived from a word meaning "fast day," which explains why many Buddhists go to temple and fast during Poyas.
Each Poya Day, has its own name and specific events it is meant to commemorate. These will be events related to Buddha and Buddhism.
Some of the key Events remembered on Poya Days include: Buddha's birthday, Buddha's enlightenment, Buddha's sending out of 60 disciples as missionaries, the personal visits Buddha made to Sri Lanka, the bringing of Buddhism to Sri Lanka by Mahinda, and the first Buddhist Council held after Buddha's death.