1801 (MDCCCI) was a common year starting on Thursday of the Gregorian calendar and a common year starting on Tuesday of the Julian calendar, the 1801st year of the Common Era (CE) and Anno Domini (AD) designations, the 801st year of the 2nd millennium, the 1st year of the 19th century, and the 2nd year of the 1800s decade. As of the start of 1801, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.