Because of its emphasis on repentance, Advent is a season of great seriousness. Advent is not a proper time for festivity, much less of partying and secular concerns. Advent is not part of the Christmas holidays, and Christians of earlier times would be shocked at the current habit of treating this as nothing more than a slightly less festive prelude to Christmas. It is the culture that drives Christmas festivity and that has served to diminish the distinctive character of Advent. It is up to the Church to maintain good worship and devotional practice during the Advent season. If we do not uphold the solemnity of Advent, no one else will.
The seasons of the liturgical year involve more than liturgical worship. The liturgical season is supposed to govern the lives of those who observe them. For this reason, anticipating Christmas activities during Advent considerably lessens the chance of our being properly prepared, by repentance, for the grace of that greater season; it also heightens the likelihood that we will fall prey to the worldly spirit that the commercial world would encourage during this time.