1624 (MDCXXIV) was a leap year starting on Monday of the Gregorian calendar and a leap year starting on Thursday of the Julian calendar, the 1624th year of the Common Era (CE) and Anno Domini (AD) designations, the 624th year of the 2nd millennium, the 24th year of the 17th century, and the 5th year of the 1620s decade. As of the start of 1624, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.