This is a dumb idea.
1. Some do not "celebrate" these holidays. A vocal majority may, but there's simply no day on the calendar where WifS can assume every paying customer is "celebrating". Making the list American-centric ignores a significant portion of the customer base. Jingoistic crap.
2. As it stands, there is no "advantage" in the schedule for any cultural sub-group. This would provide an "advantage" for americans who celebrate christmas because THEY are the only ones who care about having December 25th as a "placeholder day". The rest of us can go screw, huh?
3. As way of example, should the site be shut down for Ramadan? Chinese New Year? Yom Kippur? Bastille Day? Guy Fawkes Day? Where's the line? It's simply not acceptable to draw up a list that is convenient for the americans or the christians or the jews or any other cultural sub-set. There's NO FAIR WAY to enact this, none. And if your response is "too bad for them"...well, get an f'ing life. The world doesn't revolve around you.
4. If your commissioner is paying attention, significant events can be scheduled AROUND these holidays. Last season, the Rule 5 draft in one of my leagues was held on Dec 25th. This was accomplished by the commissioner looking at the calendar, understanding the dates, asking the world if it was OK to delay rollover by one day, setting things up so no one had to do anything on Dec 25th if they didn't have the time.
5. Most of the time, all that happens on these holidays is 3 games being simmed. If that is too onerous a schedule, you need to find a new game to play, because that's what should be happening 300 days a year if you play HBD.