1820 (MDCCXX) wis a loup year stairtin on Seturday o the Gregorian calendar an a loup year stairtin on Thursday o the Julian calendar, the 1820th year o the common auld days (ce) an anno domini (ad) designations, the 820th year o the 2nd millennium, th 20th year o the 19th hunner years, an the 1st year o the 1820s decade. As o the stairt o 1820, the Gregorian calendar wis 12 day ahead o the Julian calendar, that remained in localised uiss til 1923.