God nor Jesus ever command that his birth be celebrated.December 25th is never mentioned anywhere in the holy scriptures as being Jesus B-day.
NEITHER God nor Jesus ever commanded computers be built, nor did they ever command participation on Religious Forums. Frankly, from what I've seen, the only guys who get the "only if in the Bible" thing right are the Amish.
Besides, it actually makes sense to celebrate the birth of Jesus on or near the Winter Solstice, even if that's not his birthday. The Winter Solstice is the longest night, which marks the beginning of the Waxing Year. That is, the year the Sun Comes Back. Those of us not in the Far North don't fully understand the significance of that, so try to imagine months of little sunlight, and weeks of NO sunlight. At all.
You'd want to celebrate the Sun Coming Back, too. For a Christian, what better way to mark that hope than with a celebration of the Savior's birth during metaphorical Sunless Days?