Under the Gregorian calendar, the last year in a Century (a year divisible by 100) is not a leap year unless it is also divisible by 400. Thus, 1700, 1800, and 1900 were not leap years. The calendar was invented a few years before 1600, so next year will be only the second time in history this aspect of the Gregorian calendar will take effect. Britain and colonies didn’t adopt the Gregorian calendar until the 18th century, so this is the first Century-ending leap year in the United States.
Of course, the “exception to an exception” provisions of Gregor’s calendar will confuse a lot of people, who are sure to insist that 2000 should not be a leap year.