Māgha Pūjā day marks the four auspicious occasions, which happened nine months after the Enlightenment of the Buddha at Veḷuvana Bamboo Grove, near Rājagaha in Northern India. On that occasion, as recorded in the commentary to the Mahāsamayasutta, DN-Comm 20) four marvellous events occurred:

1. 1,250 disciples came to see the Buddha that evening without being summoned.
2. All of them were Arhantas, Enlightened Ones, and all were ordained by the Buddha himself.
3. The Buddha gave those Arhantas the principles of Buddhism, called “The ovadhapatimokha”. Those principles are: – To cease from all evil,- To do what is good,- To cleanse one’s mind;
4. It was the full-moon day.

The Buddha gave an important teaching to the assembled monks on that day 2,500 years ago called the ‘Ovādapātimokha'[1] which laid down the principles of the Buddhist teachings. In Thailand, this teaching has been dubbed the ‘Heart of Buddhism’.

Source: Wikipedia