Where the world celebrates another rotation of the earth around the sun. In most countries it is celebrated on the 31st of December. However this is an arbitrary day. A new year should really start after an equniox (either spring or autumn, this is where the length of the day and night are equal, both are 12 hours) or even after a solstice (again either summer or winter). In this case this is where the day or night is at its longest for the year.