Poson full-moon poya day (Poson Poya Day/Poson Festival) is celebrated the arrival of Buddhism in Sri Lanka in the 3rd century BC, a festival of great historical and religious significance celebrated island-wide by Buddhists. Poson, held on the full moon day of June, commemorates the occasion over 2,000 years ago when Arahat Mahinda, son of Emperor Asoka of India, converted King Devanampiyatissa to Buddhism. The main center of celebration is Mihintale, the ancient monastic complex where the royal missionary monk Mahinda perched the first sermon to the ruling king and at Anuradhapura the ancient capital, to which large crowd of pilgrims converge. Mass religious observances, illuminations pageants are part of the celebrations.
For Buddhists, this hallowed day of Poson is second only to Vesak in terms of importance. Although Poson is celebrated throughout the island, the major ceremonies are to be found in Anuradhapura and Mihintale. Long lines of devotees dressed in white climb the many steps to the top of the Mihintale hill first to the temple, and then to the dagobas that lie adorn the nearby hillocks. In addition, many devotees climb to the very top of the rock, to the spot where Arahat Mahinda delivered his initial discourses.
Many religious activities are organized during this poson period in Sri Lanka such as Sil campaigns,Bodhi Poojas,Dansalas(Freely giving foods, coffee, tea from people ), Poson devotional songs, Poson pandols( thoran) and lanterns. Lot of Poson pandols and Poson Lanterns are making Anuradhapura in Sri Lanka.
In Poson many temples are filled with Atasil devotees and pilgrims all over Sri Lanka to mark this great event. The devout, clad in pure white with no make-up or jewelry,make their way to the temple,to spend the next twenty four hours in quiet contemplation.