Nikini Full Moon Poya Day is the second Poya in the Vas season (Rainy-retreat).
It is the start of the rainy retreat for the Bhikkhu ordained monks.
Nikini Poya day celebrates the Dhamma Sangayana – the First Buddhist Council. According to Buddhist tradition, the council was held shortly after the death of the Buddha, and is dated to have taken place around 400 BCE.
At the council, what the Buddha had said, was agreed upon and recited. It was held at Sattapanni caves, Rajgriha (Bihar, India) under the patronage of the king Ajatashatru with the monk Mahakasyapa presiding and established the Tipitaka, the Buddhist scriptures.