"Maria would please me muchly by denying herself the Christmas frolic because it’s a fool’s day."

So wrote Jonathan Blanchard, president of Knox College, in 1858, in a letter to his wife Mary, as noted by Paddy O in the comments. Paddy O also says:

Back in the 1800s... very conservative Christians assumed that Christmas was in fact a pagan holiday and would not celebrate it.

The founder and first president of my very Evangelical college, Jonathan Blanchard, did not even want to cancel classes on that day....

Yet, Christian ritual is by its very nature adaptive. Unlike in the Torah, there's no liturgical prescriptions, so Christians are able to adapt within a culture, honoring their savior in culturally imbued ways. For our culture, a day of birth is the day we honor especially those we value. So, to honor the day of Christ's birth is to honor this man, to honor indeed the incarnation itself, making it a theological statement.

Following this analysis, we should approve of the early Christians who fell into the celebration of the Feast of the Sol Invictus. Right?

There's also an issue of theology and culture involved. Do Christians hide from the culture or do they interact with it? How do they interact with it? I very much like the transformative approach, shining light on a culture through culturally understood celebrations. 

I think your answer to my question needs to be yes. It's the old question of whether religions need to be kept pure and, if so, what counts as pure. Paddy goes on to stress "goodness, love, community, giving, peace, joy," so perhaps he would lean toward purity but define what is within that circle of purity to include a set of virtues that are not limited to Christianity or even to religion.

We celebrate that we are to be good, to live at peace, to sing and laugh and hope together. Christmas, much more than Valentine's Day, is really a celebration of the deepest reality of love. We wish each other love on this day, not sentimentality or cheap romance or egotistical lust, but actual deep, fulfilling, holistic love.

May your day be merry and full of life, full of love. On this day may we taste that fullness of love that is at the heart of God's work in this world, sending his son, to us, for us, with us, among us, seeking us out, because God first loved us.