1661 (MDCLXI) was a common year starting on Saturday of the Gregorian calendar and a common year starting on Tuesday of the Julian calendar, the 1661st year of the Common Era (CE) and Anno Domini (AD) designations, the 661st year of the 2nd millennium, the 61st year of the 17th century, and the 2nd year of the 1660s decade. As of the start of 1661, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.