Lay not up to yourselves treasures on earth: where the rust, and moth consume, and where thieves break through and steal. But lay up to yourselves treasures in heaven: where neither the rust nor moth doth consume, and where thieves do not break through, nor steal. For where thy treasure is, there is thy heart also. No man can serve two masters. For either he will hate the one, and love the other: or he will sustain the one, and despise the other. You cannot serve God and mammon.
Traditionally in the western Church, the Christmas season was 12 days (hence the song) from Christmas Day (December 25) to the Epiphany of the Lord (January 6). This is still more or less true on the Church calendar itself. In the U.S., however, and perhaps some other countries, the Epiphany is moved to the nearest Sunday, so Christmas lasts 9 to 14 days.
Even without this unfortunate change, there is no denying that the Church's Christmas season is radically different from the secular Christmas season, which begins in November ("officially" on Thanksgiving) and lasts until the evening of December 25 when the last Christmas special airs on TV.
This change came about primarily from the commercialization of Christmas. As I child, I remember adults at church bemoaning how our society has forgotten the "true meaning of Christmas." Now I hear people complain about store clerks wishing "Happy Holidays" instead of "Merry Christmas" as if they are committing an act of blasphemy. Perhaps they are, but against God or mammon?
Mammon is a word representing our greed and material desires. There is nothing wrong with gift giving at Christmas and material possessions in general (provided they are at the service of the good), but we must ask ourselves: if at Thanksgiving we are more focused on the Black Friday sales than giving thanks to God; if our children feel entitled to a certain toy, regardless of whether it is appropriate and good for them or if you can afford it; and if a merry Christmas revolves around them getting that toy; and if you are willing to inconveniece yourself and wait in line at a store for hours for the best deal but can't find the time to attend Mass during Advent, Christmas, or New Years; ask yourself: whom do you truly serve, God or mammon?