Christmas is a celebration that needs no introduction. It has been a Christian celebration since the 4th century in ancient Rome, and is now such a global phenomenon that it is more or less a secular celebration. However humans across the globe have been celebrating the winter solstice (shortest day of the year) for several thousands of years, and Christmas is a continuation of these festivities.
A well-known example is the Roman Saturnalia, in the name of Saturn, or Bacchus if you prefer the Greek name. During Saturnalia the conventions of the Romans were turned upside down; gambling was legalised, masters waited on their slaves. All the while candles were lit to celebrate light as it started to diminish up until the solstice. Related to this is an older observance called Brumalia, which is the same festival but lasting for a month, and wass described as being full of ‘drinking and merriment’ although the details are lost for the most part thanks to repression by the Roman Catholic Church.
Further over to Pagan Eastern Europe, there are even more variations of this popular trend, some of which have found their way into our contemporary Christmas celebrations. The most striking is the celebration of Yule, which was observed by the Finnish and Germanic pagans but then added into their Christmas celebrations once they had been converted to Christianity. The traditions that they brought over include the Yuletide log, Yuletide singing and so forth. Another common carryover is the Christmas tree itself, which has a number of potential origins but was a key feature in pagan worship before the spread of Christianity, so it is very possible that such a use of the tree was borrowed from the Slavic or Romuva faiths.
Why are there so many examples of various cultures and religions observing the Winter Solstice? For ancient cultures, the winter solstice was a major event. Not only was the world colder, the days were shorter and famine was a real problem, and so the natural response was to find answers and solace with friends and family. This is why amongst almost every winter Solstice observation there are common themes: a large feast, gifts and celebrations to commemorate that once again the worst of winter has passed, and a new year begins again. The threat of winter is greatly diminished, but the culture surrounding the solstice lives on.