What is the Old Calendar and which calendar (old or new) is most correct?
The Old or Julian calendar is used by all the Orthodox Churches for the Paschal cycle and by some for the rest of the year (except in Finland and Mexico). As a result, the feast of Nativity (Christmas) is observed on December 25 in Greece and Romania for instance but on January 7 in Russia and Serbia. In the United States, both practices can be found and coexist. For Pascha (Easter), the difference comes from the drifting of the old Julian (Roman / Imperial) calendar away from actual astronomical data over the course of the centuries and also from the fact that the Orthodox rule has always been to wait for Jewish Passover to celebrate Pascha. For the rest of the cycle, it should be admitted that the revised calendar is more astronomically accurate, which is significant for the feast of Navity (based on the symbolism of the sun of righteousness). Hence, the 6 month difference between John the Baptist and the Lord with the expression “I must decrease and He must increase” is echoed in the sun beginning its decline on June 24/25 and its increase (in length of days) on December 24/25 (not in January). However, as St. John Chrysostom observed, the issue is pastoral and cannot justify a schism between Orthodox Christians on this matter (as tragically has sometimes been the case).