The Julian calendar, which the whole Church once shared, counts a year a few minutes too long, and over the centuries its dates drifted against the seasons. In 1582 Pope Gregory XIII reformed it in the West, and the Gregorian calendar became the civil calendar of most of the world. The Orthodox churches kept the Julian calendar, and the gap between the two has grown to thirteen days.
In 1923 a congress in Constantinople proposed a Revised Julian calendar, which keeps the fixed feasts in step with the civil calendar. Constantinople, Greece, Cyprus, and Romania adopted it in 1924, Alexandria and Antioch in 1928, Bulgaria in 1968, and the Orthodox Church of Ukraine in 2023; most parishes of the Orthodox Church in America keep it too. The churches of Jerusalem, Russia, Serbia, and Georgia, Mount Athos, and others kept the Julian calendar, and keep Christmas on January 7.
The difference touches only the fixed feasts. Pascha is reckoned by the same older rule almost everywhere, so nearly all Orthodox Christians keep the same Easter. And on either calendar the feast is prepared for in the same way: forty days of the Nativity Fast before it, and the fast-free Twelve Days after it, to the eve of Theophany.