Let's look at the simple math:
In the four gospel accounts, there are a total of 89 chapters (Matthew - 28, Mark - 16, Luke - 24, and John - 21).
Matthew gives about 1 chapter to the birth narrative. Mark does not discuss it. Luke spends roughly 1.5 chapters on it. John, like Mark, does not mention Christ's birth. Therefore, out of 89 total chapters, only 2.5 are spent on the birth of Jesus Christ. I'm not suggesting that the birth of Christ is unimportant; rather, I'm pointing out that the gospel writers did not take much time to discuss it. Maybe we should think about this as we contemplate the celebrating we do.
For the sake of comparison, let's look at how much the gospel writers had to say in their passion and resurrection narratives. The starting point for each is the mention of the Passover. Matthew spends 3 chapters on it (chapters 26-28). Mark uses 3 chapters (14-16). Luke writes for 3 chapters about it (22-24). John spends a whopping 8 chapters (13-20) on the passion and resurrection of Christ. That makes for a total of 17 chapters total focusing on the death and resurrection of Jesus.
We must assume that the biblical writers took the most time on the content that they believed to be the most important. The above score (17 to 2.5) should show us that while the birth of Christ is significant, His death and resurrection are far more significant. Just the sheer number of chapters gives us a strong clue about what we should be celebrating (at any time of year).
The early church understood this. When they gathered, they celebrated the death and resurrection of Christ. They did this at least in part through the Lord's Supper. We have no indication that early Christians spent time focusing on Christ's birth.
Since the gospel writers emphasized the death and resurrection of Jesus more than anything else, shouldn't we do the same no matter what time of year it is?