Winter solstice is a very important solar term in Chinese Lunar calendar.
Being a traditional holiday as well, it is still now celebrated quite often in many regions. Early during the Spring and Autumn period, 2500 odd years from now, Winter solsticewas first determined as the Chinese traditional solar term among the total 24 terms. In Gregorian calendar, it is around Dec 22nd or 23rd.
Midwinter day is the very day in North hemisphere with the shortest day and longest night year-round. After it, daytime will become increasingly longer and the coldest clime will invade all the places on the Northern part of the globe. We Chinese always call it "JinJiu", which means once Winter solstice comes, we will meet the coldest time ahead.
That conclusion is proved well founded. According to scientific results, on Winter solstice, a right angle is formed by the sun and the tropic of carpricorn. Thus, the North hemisphere receives the least sunlight and the shortest day and longest night occur.
Ancient china did pay great attention to this holiday, regarding it as a big event. There was the saying that "Winter solstice holiday is greater than the spring festival". Nowadays, many regions still celebrate it as a big holiday. Northerners may have dumplings and raviolis that day while southerners may have dumplings made by rice and long noodles. Some places even have the tradition to offer sacrifices to the heaven and earth.