Christmas is supposed to be Christ's Mass. This is the part that bothers me because it gives people the mistaken idea that Jesus was born on the 25th of December. Those much more learned on the subject than I would say that the actual date of birth for him was somewhere roundabout March...but the masses in general do not know this. The Church, having taken over and co·opted a Pagan holiday, a pair of them technically (see subject line) because they needed a day and they liked what they were celebrating.
Interestingly enough, part of the confusion of Jesus' birthday is the fact that the ancient Christians, ironically enough, believed that the celebration of anyone's birthday was a Pagan act in and of itself. So, they didn't celebrate anyone's birthday or really track the passage of time insofar as a person's age. There may've been a head nod or something but there were no birthday cakes nor were there any giftings or anything that we attribute to birthdays these days.
While Christians of old didn't celebrate birthdays the modern ones most certainly do. Still not fond of Pagans, or anyone that doesn't worship their God, but birthdays are now hunky dory. Still you'd think that maybe they could've tried to pick the guy's actual birthday to celebrate it on, right?
Why did they pick the date they did? It's the Winter Solstice, which isn't so much Pagan in and of itself, but the date it was celebrated on isn't the same one we know of it being on today. Much as with the old Christians, the calendar system wasn't entirely the same as it is now. The Winter Solstice by the old calendar fell on the 25th of that month. When it got updated the Solstice was moved to the 18th and, for whatever reason, Christmas was left on the 25th. As to why that day, it's what it represented that caught the Christians eyes.
Sol Invictus was celebrated on the Winter Solstice. It was a Pagan holiday where the worship of Sun Gods, the day was referred to as having to do with 'The Birthday of the Unconquered Sun', was shared among multiple faiths. Most notably the Gods Elah·Gabal (Syrian) & Sol (the God of Emperor Aurelian) & Mithras (a soldier's God from Persia)...
...being the 'birthday of the unconquered' the Christians deemed it should be for Jesus as who could be more deserving of the title than him?
Saturnalia I don't know so much about. Regardless, the Christians in their usual way of stamping out Paganism (which often involved a lot of persecution and torture vack in the day) stamped out the other religions and celebrations celebrated on that day to claim it in the name of Jesus.
Oh, de humanity. The hypocrisy.
Don't you think that if Jesus' birthday is to be celebrated by the masses in general it should be actually done on the day of his actual birth and not borrowing on, or really stealing, the holidays of other faiths?