The ancient Roman holiday best known today is Saturnalia. In the late Empire, Saturnalia was a five-day festival that would have run from December 17th through December 23rd. That happens to be today.
The cultural mishmash that is modern Christmas got its start with Saturnalia. The Roman god Saturn ruled over agriculture and harvests, but more than that he represented a time in the past when abundance and peace reigned supreme.
During Saturnalia, Romans reverted back to that nostalgic time that never existed. Masters served slaves and what was normally forbidden was allowed to thrive. Publicly, the festival was celebrated by a sacrifice at the Temple of Saturn and unlike every other festival in the Roman calendar, Saturnalia was observed by everyone under Roman rule. It didn't matter where you lived or how close you were to the nearest temple of Saturn, you celebrated Saturnalia.
Where that festival fits into modern life is that a hallmark of it was that the Romans exchanged gifts with their loved ones during Saturnalia.
Human beings are today what they've always been and it's a human characteristic to express affection through a gift. The Romans turned it into a holiday and theirs is a practice modern people continue today.
The point of this is not to discount why people do what they do in 2011 or to pretend the cultural significance of the next few days isn't different now than it was then. Rather it's to show once again that everything modern westerners do is built on the people who came before us. Every aspect of our culture sits on the shoulders of our ancestors, be they genetic or cultural.
So while you're standing in line to buy gifts over the next few days, blame the Romans. But more than that, thank them for their legacy and for giving you the excuse to express your love for the people who mean the most to you.