Chinese New Year is a new moon, the second one after the winter solstice, which is why it wanders between late January and late February.

The date is calculated, not chosen, and the same reckoning sets Tết, Seollal and Losar. It triggers the largest annual human migration on the planet, scheduled by an event nobody can actually see. Leap months are slotted in every few years to stop the whole calendar drifting away from the seasons.