The Millennium celebrations were a worldwide, coordinated series of events celebrating New Year's Eve in 1999 -- 2000, marking the end of the second millennium and beginning of the new, third millennium. This also marks the ending of the 20th century and the beginning of the 21st century. Many countries held official festivities in the weeks and months leading up to the millennium, such as those organised in the USA 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 -- such as souvenir postage stamps.

Answer this question based on the passage: when did the world celebrate its most recent millenium? Let's have some stream of consciousness first.
The relevant information is: The Millennium celebrations were a worldwide, coordinated series of events celebrating New Year's Eve in 1999 -- 2000, marking the end of the second millennium and beginning of the new, third millennium. So, the answer is New Year 's Eve in 1999 -- 2000.