The story
The old story is Nián, a beast that came out of the mountains at the turn of the year to eat livestock and children, and was found to be afraid of three things: red, fire and loud noise. That is the whole visual language of the festival — red couplets on the doorframe, red envelopes, lanterns, firecrackers.
Each year carries one of twelve animals and one of five elements, so the pair repeats every sixty years. The animal is the first thing anyone asks about a birth year, and the cycle is the same one the calendar has counted in since the Han.
What the day actually is, though, is the journey. Chunyun, the travel season around it, moves several billion trips in forty days — the largest annual human migration there is — because the rule is that you go home.