The story
Là was the winter sacrifice to the ancestors and the gods of the household, held at the end of the farming year — the twelfth lunar month is still called the là month for it.
Buddhism gave the date a second reason. The Buddha, starved to a skeleton by six years of austerity, accepted a bowl of milk rice from a village girl and ate, and reached awakening that night. Temples give away congee on this day for that bowl.
The congee itself is the festival. Eight or more grains, beans, nuts and dried fruit, cooked slowly overnight, and the number of things in it is a matter of local pride.