RAYMOND J. LAWRENCE is an Episcopal cleric for 46 years, recently retired Director of Pastoral Care, New York Presbyterian Hospital, and author of numerous opinion pieces in newspapers in the U.S. and wrote the following.
Christmas was adopted by Christianity late, by some three hundred years. It was incorporated into Christianity in the 4th century, the same way Friday fish-eating was incorporated and during the same time. (Imperial Romans ate fish on Fridays to honor Venus, the goddess of love, fish being the food of love and sex.) The venerial fish-eating was simply co-opted by Christianity and given a revised rationale, namely that Jesus died on Friday, so one should abstain from eating meat on Friday.
In imperial Rome, the December 25 feast in honor of the Invincible Sun, Sol Invictus, was accompanied by the exchange of gifts, cutting of greens, lighting of candles, and public festivals commemorating new life. The sun, after all, had turned in the sky and was rising earlier and setting later, after the winter solstice.
[snip ed.]
To liberate Christmas from the clutches of Christianity would demonstrate a generosity of spirit on the part of Christians that would set a good example in these times of increasing strife between the various religions of the world.
[It certainly would improve my solstice celebration.]
turnings :: daniel berlinger 
The emphasis of celebration of Sol Invictus, the Roman sun god came towards the end of the Roman Empire. In 274 AD Roman Emperor Aurelius elevated this relatively minor god of the Roman pantheon to prime place, dedicating a temple in his honour and instituting the Solar games (held every 4 years) most likely in October (not December). The earliest and only explicit references to a one day holiday in honour of Sol’s birthday on 25th December (near the Winter solstice) is in Natalis Invicti (birthday of the invincible one – which could in fact apply to another deity) in 354 and then Julian the Apostate’s hymn to Helios (ie Sol) in 362 AD. Julian wished to reconvert the Roman Empire from Christianity to Paganism. It may have been celebrated with a procession of lights.
Even if the Natalis Invicti was to Sol (which is by no means certain as this date is 41 years after the Emperor Constantine’s conversion to Christianity), both dates occur some 18 and 26 years after the first recorded connection of Christmas to the 25th December in 336AD. See for instance http://en.wikipedia.org/wiki/Dies_Natalis_Solis_Invicti
The giving of gifts was actually associated with the Saturnalia (introduced in 213BC as a morale booster – and celebrated from 17th-23rd of December). On the otherhand, decorating with evergreens was a widespread across many cultures during winter & in Rome it was associated not with Sol Invictus or Saturnalia – but with the Roman New Year Calends (15 January).
When fixing the date to celebrate Christ’s birth the church delibrately chose a date that came between the Saturnalia and Calends (or Roman New Year on 15 Jan) so it would not be confused with these pagan celebrations – and the giving of gifts was discouraged again to avoid confusion betweent the Pagan and Christian festivals. Mention of celebrations of Jesus’ birth start appearing around 200AD (eg Cyprian mentions that this is the practice of some Egyptian Christians) though these earlier celebrations were on different days – 20 March or 6 January.
To make a simple equation between the birthday of Sol Invictus & Christmas is rather teneous – given the birth of Jesus was celebrated by at least some Christians at least 74 years before the Solar games were instituted in 274 Ad in October & the only 2 recorded instances of celebration of Natalis Invicii was 18 & 26 years AFTER 25 December was set as the date of Christmas in Rome.