Winter begins on Dec. 21 in the Northern Hemisphere. It is the shortest day and the longest night of the year. (Wikipedia)
Dec. 25 is Christmas Day, a day celebrated by Christians and non-Christians with church services, caroling, gift-giving, Christmas decorations and decorated Christmas trees.

Christmas trees have a history of not being accepted in America as late as the 1860s, because evergreens were seen as pagan symbols. German and Irish immigrants are credited with bringing about the acceptance of decorated Christmas trees. Celebrating Christmas has become more secular than religious in some ways.
Dec. 26 marks the beginning of Kwanzaa, a weeklong celebration in the U.S. that honors African heritage in African American culture. (Wikipedia). Kwanzaa was started by author Maulana Karenga (born Ronald McKinley Everett in 1941; American professor of Africana Studies, activist).
Kwanzaa was first celebrated in 1966. There are seven principles to guide followers, and each night, a candle is lit in a seven-candle holder, called a kinara, to honor each principle.

Imani is the seventh principle. It is “faith,” believing in people and “the righteousness and victory of the struggle.” Jan. 1 marks the end of Kwanzaa, celebrated with feasts and gift-giving.
I focused only on Christmas and Kwanzaa in this article, but there are other winter celebrations that occur all over the world.