The solar year, which is the basis of the Gregorian calendar used around the world, is a bit more than 365 days long. The traditional Chinese calendar, which determines the date of the Lunar New Year, is lunisolar, which means it’s based on the cycle of the moon as well as on Earth’s course around the sun. A month on this Chinese calendar is 28 days long, and a normal year lasts from 353 to 355 days. To keep the calendar in sync with the sun and the seasons, the Chinese add an extra leap month about once every three years .
Determining the date of Chinese New Year requires some complicated calculations. In most cases, it falls on the second new moon after the winter solstice. In the Gregorian calendar, the solstice is around Dec. 21. That’s why Chinese New Year typically occurs in late January or during one of the first three weeks of February.
Source: howstuffworks