Poson, also known as Poson Poya is an annual festival celebrated by Sri Lankan Buddhists, second in importance only to Wesak. Poson Poya commemorates the arrival of Buddhism in Sri Lanka in the 3rd century BC. Poson occurs on the full moon day of June every year. Poya means full moon in the Sri Lankan language.
In 236 BC, during the reign of King Devanampiyatissa, Buddhism was introduced to Sri Lanka by Arahant Mahinda,the son of Emperor Asoka and the monks who travelled from Dambadiva in India. The establishment of Buddha Sasana (an order of things governed by a Buddhist consciousness) took place with the ordination of Prince Aritta, the nephew of King Devanampiyatissa, by Arahant Mahinda.