Winter Solstice is a very important traditional solar term in the Chinese lunar calendar and also one of the traditional festivals. It is still common in many places to have customs related to the Winter Solstice festival. The Winter Solstice is also known as "Winter Festival", "Longest Day Festival", "Sub-Year" and so on. More than 2,500 years ago during the Spring and Autumn period, China already used a gnomon to observe the sun and determine the Winter Solstice. It is the first of the twenty-four solar terms to be established. The time is between December 22nd and 23rd of the Gregorian calendar each year.

The Winter Solstice is the day with the shortest daylight and the longest night in the Northern Hemisphere. After the Winter Solstice, the days gradually become longer. Ancient people described the Winter Solstice as: "the peak of yin, the beginning of yang, the sun reaches its southernmost point, the day is at its shortest, and the shadow is at its longest," hence called "Winter Solstice." After the Winter Solstice, the climate in various regions enters the coldest phase, which is commonly referred to as "entering the nine," and there is a saying in Chinese folk culture that "the coldest is during the third nine, and the hottest is during the third summer."

According to modern astronomical science, on the day of the Winter Solstice, the sun shines directly on the Tropic of Capricorn, and the sunlight is most inclined toward the Northern Hemisphere. On this day, the Northern Hemisphere has the shortest day and the longest night, and after this day, the sun gradually moves northward.

In ancient China, the Winter Solstice was highly valued. It was regarded as a major festival, and there was a saying that "Winter Solstice is as important as the New Year." There were customs of celebrating the Winter Solstice. The Book of Han says: "During the Winter Solstice, yang energy begins to rise, and the way of the ruler extends, so it is celebrated." People believed that after the Winter Solstice, the days gradually became longer, and yang energy returned, marking the beginning of a new cycle of solar terms and a lucky day that should be celebrated. According to the Jin Shi, there was an account that "During the Wei and Jin dynasties, on the Winter Solstice, envoys from all countries and officials paid their respects... the ceremony was almost as grand as that of the New Year's Day." This indicates the importance given to the Winter Solstice in ancient times.

Nowadays, some places still celebrate the Winter Solstice as a festival. In northern regions, there are customs of slaughtering sheep, eating dumplings, and eating won-ton on the Winter Solstice. In southern regions, people eat Winter Solstice rice balls and long noodles on this day. In addition, various regions have customs of sacrificing to heaven and honoring ancestors on the Winter Solstice.