As early as 2,500 years ago, about the Spring and Autumn Period (770-476 BC), China had determined the point of Winter Solstice by observing movements of the sun with a sundial. It is the earliest of the 24 seasonal division points. The time will be each December 21 or 22 according to the Gregorian calendar.
The Northern hemisphere on this day experiences the shortest daytime and longest nighttime. After the Winter Solstice, days will become longer and longer. As ancient Chinese thought, the yang, or muscular, positive things will become stronger and stronger after this day, so it should be celebrated.
The Winter Solstice became a festival during the Han Dynasty (206 BC-220 AD) and thrived in the Tang and Song dynasties (618-1279). The Han people regarded Winter Solstice as a "Winter Festival", so officials would organize celebrating activities. On this day, both officials and common people would have a rest. The army was stationed in, frontier fortresses closed and business and traveling stopped. Relatives and friends presented to each other delicious food. In the Tang and Song dynasties, the Winter Solstice was a day to offer scarifies to Heaven and ancestors. Emperors would go to suburbs to worship the Heaven; while common people offered sacrifices to their deceased parents or other relatives. The Qing Dynasty (1644-1911) even had the record that "Winter Solstice is as formal as the Spring Festival", showing the great importance attached to this day.