Correction:
We do not celebrate Nativity on January 7th, we celebrate Nativity on December 25th. The Roman pope changed the calendar in the 1500s, skipping ahead 13 days. Since all other Christian faiths came out of Rome, all protestant faiths also celebrate on the “new calendar.”
We celebrate Nativity on December 25th, according to the old calendar, which appears to be January 7th according to the secular calendar.
Christ is Born!
It's 13 days now. In 1582 it was only 10 days.
Someone is going to read this and suppose that it's just another case of gross Papal hubris and overreach. In fact, God -- not the Pope -- made the solar year 365 days, 5 hours, and about 47 minutes. The Julian calendar assumes a solar year of 365 days and 6 hours, exactly.
Between Nicaea and 1582, that 13 minute per year difference added up such that the observed astronomical vernal equinox was occurring 10 days earlier than Nicaea put it. (More accurate observations have Nicaea being about a day late, to boot, but we still use the Nicaean date of 21 March.)
The Gregorian calendar adjusted things back to where they were at the time of Nicaea. The Julian calendar had already slipped 4 days between the Our Lord's birth and Nicaea. Christmas, at December 25th, originally fell at or almost on the winter solstice. By Nicaea, the winter solstice was more like December 21, where it is (on the Gregorian calendar) today.
We should not deify the old calendar. It was produced by the then pagan Pontifex Maximus, Julius Caesar. He had attempted to replace the older Roman lunar calendar with a purely solar one. Unfortunately, his math was a little off. Pope Gregory just adjusted it so that the calendar year matched the actual solar year, which is what Caesar had attempted with the calendar in the first place.
The calendar is neither Scripture nor revelation. I have never really understood why the Orthodox have wanted to stick with a flawed pagan civil calendar that is mathematically incorrect.
Indeed He is Born!!!!
Merry Christmas to all Old Calendar Orthodox Christians!!!!