Māgha Pūjā is an important Buddhist festival celebrated on the full moon day of Māgha in Cambodia, Laos, Thailand and Sri Lanka and on the full moon day of Tabodwe in Myanmar. The spiritual aims of the day are not to commit any kind of sins; do only good; purify one’s mind. Māgha Pūjā is a public holiday in Cambodia, Laos, Myanmar, Sri Lanka and Thailand and is an occasion when Buddhists tend to go to the temple to perform merit-making activities.

The third lunar month is known in the Thai language as Makha (Pali: Māgha); Bucha is also a Thai word (Pali: Pūjā), meaning “to venerate” or “to honor”. As such, Makha Bucha Day is for the veneration of Buddha and his teachings on the full moon day of the third lunar month.

Māgha Pūjā day marks the four auspicious occasions occurring at the Veḷuvana bamboo grove, near Rājagaha in northern India ten months after the enlightenment of the Buddha. On that occasion, as recorded in the commentary to the Mahāsamayasutta, DN-Comm 20) four marvellous events occurred:

1,250 disciples came to see the Buddha that evening without being summoned.
All of them were Arahants, Enlightened Ones, and
All were ordained by the Buddha himself: Ehibhikkhus.
It was the full-moon day.
On this occasion the Buddha gave those Arahants the principles of Buddhism, called “The ovadapatimokha”.[5] Those principles are: To cease from all evil; To do what is good; To cleanse one’s mind. In Thailand, this teaching has been dubbed the “heart of Buddhism”. Wikipedia