If, as you claim, the association of 25 to "advent" is primarily commercial, that's much more of a reason to avoid that association. In any case it's very culture-specific. In many countries, including mine, Christmas Eve is the "main" event that people look forward to, and the number of "advent days" in calendars and such is 24. On the other hand, ecclesiastically there are four Advent Sundays, and the number of days is thus variable and also not really pertinent.