It was during the reign of King Devanampiya Tissa one of the ancient rulers of Ceylon or Sinhale that Buddhism "Officially" arrived and was established within the island. This king was ruling at the ancient capital of Anuradhapura from 307 BC to 267 BC.
Ashoka during this period. The primary source for King Devanampiyatissa's notable period in the island's history is Mahavamsa. The more ancient chronicle on which the Mahawansa is based is called Dipavansa.
Arahant Mihindu arrived with five other Arahant Bhikkhus and one Anagami Upasaka. He was the son of Emperor Ashoka of India. With this Message of the Dhamma, the King stopped hunting innocent animals. Instead even hospitals were started for animals. The island saw its golden era during the immediate period after Arahat Mihindu’s arrival. Such as had never been before nor would it be likely to happen again. Such was the impact His arrival created on society.
The island nation prospered under King Devanampiyatissa’s rule. Agriculture flourished , mainly horticultural products with the help of huge lakes which were man- built in the dry zone to aide paddy cultivation which earned the island the nick name “Granary of the East”. The people were simple, gentle and easily satisfied as a result of the Dhamma that engulfed them, and they were a happy lot. As a result of the populace being contented, the country saw its economy booming very strong and rich. The produce were spread amongst all the people, because, nobody took what was not needed by them. This helped circulate the resources immensely, and catered to all the needs of the public. Unlike the commercial economies of today, where people are never contented with what they have. They buy and buy and buy, what they don't really need too, and however much a country produces it is not enough. Thousands became enlightened and there were many places of teaching and practice of meditation. During the first Dhamma discourse itself, namely the Culla Hatthi Padhopama Sutta, (simile of the Elephant foot print), the consort of King Devanampiyatissa, became a Stream Winner (Sothapanna or the 1st stage in enlightenment) and expressed her wish to become ordained.
When this wish was conveyed to the Arahat Mihindu, He sent forth messengers to His Sister Sanghamitta. She was also an Arahat. Thus she arrived in Sri Lanka to ordain the consort of King Tissa, Queen Anula, who later became an Arahat herself.
She came to Sri Lanka together with several other nuns to start the nun-lineage of Bhikkhunis (a fully ordained female Buddhist monastic) at the request of King Tissa to ordain queen Anulā and other women of Tissa's court at Anuradhapura who desired to be ordained as nuns after listening to the Dhamma talks given by Arahat Mihindu. We can thus fathom that these talks were so inspirational. Arahat Bhikkuni Sanghamitta brought a sapling from the Bodhi Tree at Bodhgaya as a gift to Lanka.
After Sanghamittā’s contribution to educate the women in Lanka thus, by establishing the 'Bikhhunī Sangha' or 'Meheni Sasna' (Order of Nuns), her name became synonymous with the 'Buddhist Female Monastic Order of Theravāda Buddhism', which got rooted not only in Sri Lanka but also in Burma, China and Thailand, in particular.
The Bodhi tree, the sapling which she brought was planted in Anuradhapura, and is still surviving. This day is also celebrated every year on the Full Moon day of December the day she arrived in the island some 2000 years ago. This day is known as Unduvap Poya and "Sanghamittā Day."