Today Shawneen spoke to us about celebrations of the Winter Solstice now and in the past.  The word is a combination of the latin terms of sōl sun + a variant stem of sistere to makestand In other words it means the time of making the sun stand still.  Because the earth rotates on a tilt, the day where the axle of the tilt is pointed away from the sun has the shortest amount of sunlight of any day (at least in the Northern Hemisphere.)  As a result most primitive cultures had a celebration to mark the day when the sun stops moving to the south as dawn breaks and starts moving back north again.  As Shawn pointed out, Axial tilt is the reason for the season.

There are three prominent Neolithic sites that demonstrate that primitive Europeans were devoted to marking the day when the sun would begin its march back north.  Stonehenge in Wiltshire, England is the most famous.  However Maeshowe on the island of Orkney, Scotland and New Grange in County Meath Ireland are (probably) both older (2,500 to 3,000 B.C.E.).

There are several reasons why we humans would want to mark such an event.  Predicting the event enables humans to feel like they have some control over their world.  It gives the priestly class an opportunity to perform a ceremony which appears to the common people to make the sun reverse the process of diminishing daylight.  In an agricultural society, sun is necessary to grow things.  Things that stayed green through the winter would have magical properties ascribed to them; the holly and the pine.  At this time of year, the harvest is in and the hard work of farming is not demanding so much time.  In most cultures there is some provision for sharing the bounty across the community if some members of the community are in need.  We continue that tradition today although much of that has been transferred to the Thanksgiving celebration.

The Roman celebration of Saturn (Saturnalia) took place at the Solstice and by 374 C. E. the birth of Jesus was being celebrated at the same time.  All of the celebrations were taken together and bits and pieces ended up in our modern Christmas celebration.  Joy of Community... Return of the light... celebration of survival of the long night... reverence for the green things... the return of prosperity... all have their roots in ancient celebrations of the Solstice. 
This is a photo of Newgrange in Ireland: