For me the main Christmas holiday is 24th and I'm generally away from my proper gaming computer for at least a week, qq, make anything I would participate not happen during that.
Seriously, sure, it's kind of foul play to attack another group of players that has a big holiday when you don't, but such is life. If you want to start taking into account times when considerable amounts of playerbase have big holidays you'll have a huge list of stuff like that. Midsummer is big thing in Finland and probably some other European countries. Thanksgiving is big in the US (and Canada, no clue?) but isn't celebarated in Europe.
Easiest way to deal with this is to either a) prepare for things like this with a diverse playerbase (even if current NA-EU split makes having a clan with both continents represented worse than before) b) just strike those who do this on their holidays. The Orthodox holidays follow Julian calendar while most of Europe and US follow Gregorian calendar so the (religious) holidays tend to be on different dates anyway.