On New Year's Eve 1999, what date change did the Millennium celebrations mark?

The Millennium celebrations on New Year's Eve 1999 marked the date change to January 1, 2000.

The event was celebrated worldwide as the arrival of a new millennium, even though the calendar issue was more complicated than many headlines suggested. In the Gregorian calendar, the first millennium began with year 1, not year 0. Therefore, the second millennium technically ended on December 31, 2000, and the third began on January 1, 2001.

Nevertheless, the transition from 1999 to 2000 was culturally treated as the millennium because the odometer-like change from 19xx to 20xx was visually and symbolically powerful. Major cities staged concerts, fireworks, illuminated landmarks, and countdowns. The celebrations also brought attention to the Y2K computer problem, since some systems stored years using only two digits.

This distinction explains why both 2000 and 2001 appear in discussions of the millennium. The New Year's Eve celebrations occurred at the start of 2000; the strictly numbered millennium began one year later.