The day commemorates a time when 1,250 enlightened monks, disciples of the historical Buddha, spontaneously came together to pay respect to the Buddha. The day is called “Fourfold Assembly” Day because
All the monks were arhats.
All the monks had been ordained by the Buddha.
The monks came together as if by chance, without any planning or prior appointment
It was the full moon day of Magha (March).
When the monks were assembled, the Buddha delivered a sermon in which he asked the monks to do good, to abstain from bad action, and to purify the mind.
The word sangha refers to the community of Buddhists, either all Buddhists everywhere or a particular fellowship of Buddhists. Although sangha can refer to both monks and laypeople, Magha Puja Day is called “Sangha Day” because it is a day to show appreciation to the monastic sangha.
On this day laypeople gather at temples, usually in the morning, bringing with them offerings of food and other items for the monastics. Magha Puja observations usually feature processions, sometimes by candlelight. Observers walk around a shrine or Buddha image or through a temple three times, once for each of the Three Jewels – The Buddha, the Dharma, and the Sangha.
In Thailand, this same holiday is called Makha Bucha and is observed about a month earlier.