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.