It was never explained better than in Charles Schulz’s classic TV cartoon from 1965, A Charlie Brown Christmas. As you watch this, recall that Schulz had to fight the executives at CBS to get this bit included.

From the link:

[It] was written over a period of several weeks, and produced on a small budget in six months. In casting the characters, the producers took an unconventional route, hiring child actors. The program’s soundtrack was similarly unorthodox, featuring a jazz score by pianist Vince Guaraldi. Its lack of a laugh track (a staple in US television animation in this period), in addition to its tone, pacing, music, and animation, led both the producers and the network to predict the project would be a disaster. However, contrary to their collective apprehension, A Charlie Brown Christmas received high ratings and acclaim from critics.

It has never stopped running since then.