Why are there two days for celebrating Christmas?
Christmas is always celebrated on December 25! But which day is December 25?
Some Orthodox celebrate Christmas on December 25 of the older calendar ("Julian Calendar"), some celebrate on December 25 of the revised calendar (secular or "Gregorian Calendar"). December 25 in the Julian Calendar is January 7 of the Gregorian Calendar.
The Julian Calendar has a leap year every 4 years. The current secular calendar (Gregorian Calendar) does not have a leap year in those years ending in "00" where the hundreds-part is not divisible by 5 -- and thus 1900, 2100, 2200, ... are not leap years. In both calendars the year 2000 is a leap year.
The Gregorian Calendar (sometimes called the Revised Julian Calendar) was adopted as the secular calendar in 1585 in parts of Western Europe, in 1753 in England and North America, and 1923 in Russia.
George Washington (1789-1797) was born on February 11 old calendar (i.e., before 1753). That is why his birthday is celebrated on February 22 ("Presidents Day", new calendar). In his times the difference between the old and new calendars was 11 days.
The two days when Christmas is celebrated drift apart an extra three days every 400 years. December 25 in the Julian Calendar becomes January 8 in the secular (Gregorian Calendar) starting in the year 2100.
The Julian Calendar is named after Julius Caesar. The Gregorian Calendar is named after Pope Gregory.
The Orthdox Church in America also commemorates Creation on September 1st, however, I've yet to hear any Orthodox suggest that's when Creation occurred. It's just when we commemorate it.
The supposed theologic fuss over commemorating the birth of Christ stems from certain Christians who merely can't stand the concept of any Christian not embracing their every interpretation as the Gospel truth.
Around here, we call them the followers of YOPIS (Your Own Personal Interpretation of Scripture). Not only can their posts be ignored, it is also spiritually advisable.