The Binara poya (full moon day) is the poya day in the month of September and falls within the four month Vassana period.
The Vassana period is the rainy season when bhikkhus and bhikkhunis stay in one place, observe the disciplinary code laid down by the Buddha and concentrate on meditation and other religious activities. They do not go begging for food (pindapatha) but the dayakayas (lay patrons) will look after them.
The Bhikkhuni Sasana also originated on a Binara Full Moon Day and the first bhikkhuni to be ordained was Queen Prajapathi Gothami. The annual religious festivals of the Girihandu Seya, Mahiyanganaya, and the Somawathi Chaithya are held during the month of Binara.
Maha Prajapathi Gothami and the order of the Bhikkhunis 
The order of the Bhikkhunis was founded by the Buddha at the insistence of his maternal aunt and step mother, Maha Prajapathi Gothami. She was the younger sister of the Buddha’s mother, Queen Mahamaya Devi. After the Queen’s death King Sudddodhana made Maha Prajathi Gothami his Queen and she brought up Prince Siddhartha.
After the death of King Suddhodana, Maha Prajapathi Gothami asked the Buddha to ordain her as a bhikkhuni and he refused her thrice and did not give any reason for the refusal.
The Buddha left Kapilavasthu and went to Vesali. A determined Maha Prajapathi Gothami shaved her head, donned yelow robes and accompanied by a large number of Sakyan ladies walked to Vesali to meet the Buddha and again ask for ordination. She waited outside the temple. Ananda Maha Thera saw her and asked why she was there. She said she was there as the Buddha has not given permission for women to be ordained.
Ananda Maha Thera offered to ask the Buddha and did so. He asked three times and was refused. He, then asked the question in a different way. The Buddha said that if Maha Prajapathi Gothami was willing to accept eight strict rules related to the Vinaya, she and the others could be ordained.
Maha Prajapathi Gothami accepted the conditions and was ordained. The other Sakyan ladies too were ordained. Soon after, Maha Prajapathi Gothami became an Arahant.
Girihandu Seya
The Girihandu Seya is a stupa built by two merchant brothers Tapassu and Bhalluka in Thiriyaya, on the Tricomalee-Pulmoddai Road, 45 km north of Trincomalee. It is considered to have been built during the life time of the Buddha. During the 50th day after his Enlightenment the Buddha was meditating under the Rajayatana ‘Kiripalu' tree when Tapassu and Bhalluka offered him alms and asked him to give them a relic to worship. The Buddha gave them kesa dathu (hair relic) and they placed it in a golden casket. Tapassu and Bhalluka were the very first devotees to accept the Buddha and Dhamma. The order of the Sangha was not established at this time.
Tapassu and Bhalluka always carried the sacred hair relic with them. They came to Sri Lanka and travelled towards the Yang Oya near Trincomalee. They kept the golden casket with the hair relic atop a nearby hill and covered it with a white cloth. When they wanted to take the casket away they could not as it was embedded in that spot. They built a small stupa to house the relic and went back to India. Originally named the Girikandi Chaitya it later became known as the Girihandu Seya.
According to the Thiriyaya stone inscription (near the Girihandu Seya), the stupa was completed 75 days after the enlightenment of the Buddha.
Mahiyanganaya
The Buddha’s first visit to Sri Lanka was to Mahiyangana in the ninth month after his Enlightenment on the Duruthu Full Moon day to bring peace between the Yakkha and Nagas. The Buddha preached the Dhamma and the tribe became devotees.
The Yakkha chieftain Sumana Saman (now the deity Sumana Saman, one of the four guardians of the island) who was also there. He requested the Buddha for a relic and was given some locks of hair by the Buddha. The deity built a stupa or chaithya and enshrined the relics there. This was the Mahiyangana Chaithya. After the Buddha’s Parinirvana an Arahant named Sarabhu brought the Buddha’s left shoulder bone recovered from the funeral pyre which was also enshrined here and the Stupa was enlarged to a height of 18 feet (5.5 m).
Somawathi Chaithya
Somawathiya (Somawathi Chethiya) was built in the 2nd century BC and enshrines the right tooth relic of the Buddha.
Historical records show that princess Somawathi (sister of King Kavantissa) and her husband prince Giri-aba lived here.
According to chronicles, Prince Giri-aba and Princess Somawathi (sister of King Kavanthissa) lived in 'Somapura' on the beds of Mahaveli River. Prince Giriaba constructed Somawathi Chethiya for his wife to engage in religious activities. Upon completion of the Stupa, the prince requested a bhikku named Mahinda for some 'dathu' to be kept in the relic chamber of the stupa. and he gave him the right tooth relic of the Buddha.
When the stupa and the viharas were completed the prince and princess handed over the temple to Mahinda Maha Thera.