813 (DCCCXIII) was a common year starting on Saturday of the Julian calendar, the 813th year of the Common Era (CE) and Anno Domini (AD) designations, the 813th year of the 1st millennium, the 13th year of the 9th century, and the 4th year of the 810s decade. As of the start of 813, the Gregorian calendar was 4 days ahead of the Julian calendar, which was the dominant calendar of the time.