popa,
Your analysis is not correct. First, December 25, 2010 on the Julian calendar is January 7, 2011 on the Gregorian calendar, not January 6. And indeed some Orthdox Christians will be celebrating Christmas on January 7. However, in 1923 most Orthodox churches did adopt a calendar similar to the Gregorian and therefore they will celebrate Christmas on December 25. (I have read that this remains controversial.) January 6 is a different Christian holiday, Epiphany. Armenian Christians celebrate Christmas on that day.
I don’t understand why we are allowing ANY Jewish practice to depend on the date of a non-Jewish holiday.