Orthodox Christians, numbering around 250 million globally, celebrate Christmas on January 7 due to their adherence to the Julian calendar. This calendar, established in 46 BC, differs from the widely adopted Gregorian calendar introduced in 1582. The Julian calendar's slight overestimation of the solar year's length caused it to drift out of sync with the seasons over centuries, leading to the 13-day difference observed today. Traditions include fasting and specific observances in Eastern Europe, Africa, and the Middle East.