https://197lines.com/christmas-is-celebrated-today-in-the-country-a-different-date-than-25-december-because-the-russian-orthodox-church-uses-the-old-julian-calendar-for-the-days-of-religious-celebration
Christmas is celebrated today in the country, a different date than 25 December because the Orthodox Church, representing around 260 million Christians, still uses the ancient "Julian" calendar to mark religious festivities