Observance day (Uposatha)

The four monthly holy days which continue to be observed in Theravada countries - the new moon, full moon, and quarter moon days. Known in Sri Lanka as Poya Day.

Rains retreat (Pavarana Day)

This day marks the conclusion of the Rains retreat (Vassa). In the following month, the kathina ceremony is held, during which the laity gather to make formal offerings of robe cloth and other requisites to the Sangha.

Robe offering ceremony (Kathina Ceremony)
The Kathina is held on any convenient date within one month of the conclusion of the three month rains retreat season (Vassa) for the monastic order. It is the time of the year when new robes and other requisites may be offered by the laity to the monks and nuns.

Rains retreat (Anapanasati)
At the end of one rains retreat (Vassa), the Buddha was so pleased with the progress of the assembled monks that he encouraged them to extend their retreat for yet another month. On the full-moon day marking the end of that fourth month of retreat, he presented his now-famous instructions on mindfulness of breathing (anapanasati).

Buddhist New Year
In Theravadin countries, Thailand, Burma, Sri Lanka, Cambodia and Lao, the new year is celebrated for three days from the first full moon day in April. In Mahayana countries the new year starts on the first full moon day in January. However, the Buddhist New Year depends on the country of origin or ethnic background of the people.

Fourfold assembly or sangha day (Magha Puja Day)
Magha Puja Day takes places on the full moon day of the third lunar month (March). This holy day is observed to commemorate an important event in the life of the Buddha. This event occurred early in the Buddha's teaching life.

After the first rains retreat (Vassa)

At the Deer Park at Sarnath, the Buddha went to Rajagaha city where 1250 Arahats, (Enlightened saints) who were the Buddha's disciples, without prior appointment, returned from their wanderings to pay respect to the Buddha. They assembled in the Veruvana Monastery with the two chief disciples of the Buddha, Ven. Sariputta and Ven. Moggalana. The assembly is called the Fourfold Assembly because it consisted of four factor.

1. All 1,250 were Arahats
2. All of them were ordained by the Buddha himself
3. They assembled by themselves without any prior call
4. It was the full moon day of Magha month (March)