Here's something to ponder on:
In Acts 15, it clearly says to abstain from food sacrificed to idols. But in Corinthians (don't remember the exact chapter), Paul says to Christians that they should eat whatever food non-believers offer them, not inquiring of its source - whether or not it was previously offered to idols. How can that be? This utterly demolishes JWs 'origin's matter' argument. Paul was clearly showing that origins do not matter! What matters is intent and effect on others. A Christian can eat food sacrificed to idols provided that:
1. They eat such a meal with a clear conscience, treating it as any other mundane meal and not as some kind of religious observance to the god it was offered to.
2. Their being seen eating such a meal would not stumble someone with a weaker conscience.
Interestingly, there is another place in Acts where the abstain from blood and abstain from food sacrificed to idols is mentioned. In that passage is actually gives a reason for these abstentions - and the reason is very insightful and interesting. The writer says words to the effect: 'Because Moses is read in the synagogues every Sabbath'. It seems the author was implying that some of these abstentions are necessary only to avoid stumbling Jews familiar with the mosaic law. (2. above) Arguably this applies to the the command to abstain from blood. Because going back to Paul's advice, how is a Christian to know that the meat offered to them by the non-believer was properly bled? Or how do they know that blood wasn't part of the ingredients in preparing the meal? Remember: Paul said they're to just eat and not ask!
So apply Paul's principles principles to Christmas:
What is the intent of those who celebrate Christmas? Are they doing so with religious devotion toward Mithra or Saturn or some other pagan god? No! They celebrate with a clean conscience treating it as just a fun secular holiday or as a commemoration of the birth of Jesus. Of course, if they engage in gluttony or over drinking then that would be wrong from a Biblical standpoint. But that would be a matter of how a person celebrates Christmas rather than whether or not it is allowable to celebrate it, period. One does not have to engage in gluttony and overdrinking to celebrate Christmas and many don't.
Will others be stumbled by seeing you celebrate Christmas? If so don't celebrate it in their view. That can mean celebrating it secretly or not at all, depending on your circumstances.
But the takeaway from Acts and Paul which is quite in contrast to Watchtower, is that God is not some petty deity who stigmatizes things based on origins. Rather, he looks at the conscience and intent of the person involved and if their actions stumble others.