Winter solstice is one of the eight festivals in a year. In ancient times, there was a custom of "eight festivals" to worship the gods and ancestors. Winter solstice is regarded as a big holiday in winter. There is a folk saying that winter solstice is as big as a year, so the ancients called it "sub-year" or "off-year".

The summer solstice is the northernmost day of the sun in a year and the limit of the sun's northward movement. The solstice in summer is the turning point of the sun. After this day, it went back, and the direct sunlight point began to move south from the Tropic of Cancer (23 26' north latitude), and the days in the northern hemisphere became shorter day by day.

At the same time, for the Tropic of Cancer and its northern areas, the summer solstice is also the highest day at noon in a year. At this time, the southern hemisphere is in the middle of winter.

For example, the daily length of Haikou in Hainan is a little more than 13 hours, Hangzhou is 14 hours, Beijing is about 15 hours, and Mohe in Heilongjiang can reach 17 hours.