The winter solstice is the moment the Sun is the farthest away from us that it will be all year, falling between December 20th and 23rd each year in the Northern Hemisphere. It is either the shortest day or longest night of the year, (not neccessarily the darkest). It lasts only an instant, but we've long made a day and night of it. Cultures all over the world have celebrated it throughout history. Each celebration varies, but most focus on re-birth, some taking it to mark the beginning of winter, some teh middle. The word, from the Latin, means 'Sun stand still'.
Many mythologies formed around the event and the astronomical observances made on the date. For example, in the Northern sky, the three stars in Orion's belt align with Sirius, the brightest star in the Eastern sky, and indicate where the Sun will rise the following morning. Up until this point, the Sun has made a decreasing arc in the South. From this point forward, it stops its decline, resulting in 3 days of less daylight than any other time of the year. It then begins its ascent again and the days start to lengthen again. For ancient people, this was a very significant thing, (no phone, no lights, no motorcars, so the Sun was an even bigger deal than it is now). Many cultures, logically, described this as the Sun being re-born, a return to a time of light after one of great darkness.
Archaeological discoveries support the dates importance in cultures from Neolithic and Bronze age times. Stonehenge, New Grange (Ireland) and other monuments seem to have been aligned precisely to reflect it. Its' importance may have been because in the winter, the life of communities was not assured. Starvation was the norm, (the time from January - April was known as the starving time), as plant life was far from abundant (and cold probably made hunting more difficult as well). Survival depended not only upon the planning that took place in the 9 months preceeding, but on the successful preservation of the stores. In climates where teh weather was less harsh, the holiday was not as significant.
Feasting was a standard part of the celebratino because on this date, livestock were slaughtered (to save feeding them during winter, in part), making it the only time when fresh meat could be enjoyed. Wine and beer, started several months previously, had reached fermentation by the Solstice, making it a time of drinking as well. Celebrations tended to begin at mindnight, dawn or on the previous evening, (sort of like starting celebrations on Christmas Eve).
We see connections between the celebrations of the re-birth of the Sun God and the birth of Jesus at Christmas, (indeed, Christians deliberately incorporated some of these to make conversions smoother and more successful). It was also a holiday that related to our New Years' celebrations, when not only the Sun but the year itself was seen as re-born. Calendars were based on the date and life-death dieties celebrated along with new beginnings. In some celebrations, like the Saturnalia, roles were reversed for the day, reflecting the fact that each thing in society, within people etc, contains its' opposite.
Along the vein of celebrating the opposite within things, festivals at this time of great darkness, cold and isolation featured evergreens, fires, feasting, gathering with others, dancing, and singing, (its' beginning to look a lot like Christmas). Doing these things were beleived not only to stave off teh grim feelings that the cold and dark, anticipation of the 'starving months' brought on, but to prevent or at least temper their effects in the coming months. Sacrifice, (of people and animals), was standard, perhaps because cultures hoped that if some things were offerred voluntarily to the Gods, less would be taken.