The millennium celebrations were a worldwide, coordinated series of events to celebrate and commemorate the end of 1999 and the start of the year 2000 in the Gregorian calendar. The celebrations were held as marking the end of the 2nd millennium, the 20th century, and the 200th decade, and the start of the 3rd millennium, the 21st century, and the 201st decade. Countries around the globe held official festivities in the weeks and months leading up to the date, such as those organised in the United States by the White House Millennium Council, and most major cities produced firework displays at midnight. Equally, many private venues, cultural and religious centres held events and a diverse range of memorabilia was created, including souvenir postage stamps.

A millennium is a period of one thousand years, sometimes called a kiloannum (ka), or kiloyear (ky). Normally, the word is used specifically for periods of a thousand years that begin at the starting point of the calendar in consideration and at later years that are whole number multiples of a thousand years after the start point. The term can also refer to an interval of time beginning on any date. Millennia sometimes have religious or theological implications.