1631 (MDCXXXI) was a common year starting on Wednesday of the Gregorian calendar and a common year starting on Saturday of the Julian calendar, the 1631st year of the Common Era (CE) and Anno Domini (AD) designations, the 631st year of the 2nd millennium, the 31st year of the 17th century, and the 2nd year of the 1630s decade. As of the start of 1631, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.