Advent is the liturgical season dedicated before all others to hope or confident expectation of Christmas and so having a Christmas party in advent is directly opposed to the whole idea of the season.
Secular- plastic reindeer- consumer-winter goodwill-holiday Christmas is entirely over after Christmas, and so we celebrate it in advance to get a few days of pleasure in before the everything dies.
All this is a good education in the sorts of goods:
For the more perfectly the sovereign good is <!—->possessed<!––>, the more it is <!—->loved<!––>, and other things despised: because the more we <!—->possess<!––> it, the more we know it. Hence it is written (Sirach 24:29): “They that eat me shall yet hunger.” Whereas in the desire for wealth and for whatsoever temporal <!—->goods<!––>, the contrary is the case: for when we already <!—->possess<!––> them, we <!—->despise<!––> them, and seek others: which is the sense of Our Lord’s words (John 4:13): “Whosoever drinketh of this water,” by which temporal <!—->goods<!––> are signified, “shall thirst again.” The reason of this is that we realize more their insufficiency when we <!—->possess<!––> them: and this very fact shows that they are imperfect, and the sovereign good does not consist therein.