Why exactly do we celebrate New Years?

Traditionally I suppose it has to do with the passing of the shortest day of the year and the beginning of the move toward spring, planting, the next year's harvest, etc. The whole concept of the "rebirth" of the Earth. I do suspect it derives mainly from celebrations in the Northern Temeperate Zone and spread with Western Culture. Doesn't make much sense along the Equator and in the Southern Temperate Zone the seasons would be reversed.