The Magha Puja is also called the Fourfold Assembly or Sangha Day.
It is a day when Buddhists remember four special things:
Buddhists remember the day when Buddha addressed a meeting of 1250 Arahantas at Veruvana Temple in Rajgarh city
Buddhists remember that each Arahantas was individually ordained by the lord Buddha himself
Buddhists remember that each Arahantas came on his own initiative and without notification
Buddhists remember that it was a day with a perfect full-moon
Then they think about 3 aims:
Not to commit any kind of sins
Only doing good
Purifying one’s mind