Quick Answer: 1621 was a common year starting on Friday of the Gregorian calendar and a common year starting on Monday of the Julian calendar, the 1621st year of the Common Era and Anno Domini designations, the 621st year of the 2nd millennium, the 21st year of the 17th century, and the 2nd year of the 1620s decade. As of the start of 1621, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.