A millennium (plural millennia or millenniums) is a period of one thousand years,  sometimes called a kiloannum (ka), or kiloyear (ky). Sometimes, the word is used specifically for periods of a thousand years that begin at the starting point (initial reference point) of the calendar in consideration (typically the year "1") 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 (see millenarianism).
The word millennium derives from the Latin mille, thousand, and annus, year.
There was a public debate leading up to the celebrations of the year 2000 as to whether the beginning of that year should be understood as the beginning of "the" new millennium. Historically, there has been debate around the turn of previous decades, centuries, and millennia. The issue arises from the difference between the convention of using ordinal numbers to count years and millennia, as in "the third millennium", or using a vernacular description, as in "the two thousands". The difference of opinion comes down to whether to celebrate, respectively, the end or the beginning of the "-000" year. The first convention is common in English-speaking countries, but the latter is favoured in, for example, Sweden (tvåtusentalet, which translates literally as the two thousands period).
Those holding that the arrival of the new millennium should be celebrated in the transition from 2000 to 2001 (i.e., December 31, 2000, to January 1, 2001) argued that the Anno Domini system of counting years began with the year 1 (There was no year zero) and therefore the first millennium was from the year 1 to the end of the year 1000, the second millennium from 1001 to the end of 2000, and the third millennium beginning with 2001 and ending at the end of 3000.
Popular culture supported celebrating the arrival of the new millennium in the transition from 1999 to 2000 (i.e., December 31, 1999, to January 1, 2000), in that the change of the hundreds digit in the year number, with the zeroes rolling over, is consistent with the vernacular demarcation of decades by their 'tens' digit (e.g. naming the period 1980 to 1989 as "the 1980s" or "the eighties"). This sometimes referred to as "the odometer effect".  Adding to its cultural significance, the "year 2000" had been a popular phrase referring to an often utopian future, or a year when stories in such a future were set. There was also media and public interest in the Y2K computer bug.
A third position was expressed by Bill Paupe, honorary consul for Kiribati: "To me, I just don't see what all the hoopla is about ... it's not going to change anything. The next day the sun is going to come up again and then it will all be forgotten."  And even for those who did celebrate, in astronomical terms, there was nothing special about this particular event. 
Stephen Jay Gould, in his essay Dousing Diminutive Dennis' Debate (or DDDD = 2000) ( Dinosaur in a Haystack), discussed the "high" versus "pop" culture interpretation of the transition. Gould noted that the high culture, strict construction had been the dominant viewpoint at the 20th century's beginning, but that the pop culture viewpoint dominated at its end. 
The start of the 21st century and 3rd millennium was celebrated worldwide at the start of the year 2000. One year later, at the start of the year 2001, the celebrations had largely returned to the usual ringing in of just another new year,  although some welcomed "the real millennium", including America's official timekeeper, the U.S. Naval Observatory,  and the countries of Cuba  and Japan. 
The popular  approach was to treat the end of 1999 as the end of "a millennium" and to hold millennium celebrations at midnight between December 31, 1999, and January 1, 2000, with the cultural and psychological significance of the events listed above combining to cause celebrations to be observed one year earlier than the formal date. 
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- List of calendars
- List of decades, centuries, and millennia
- Millennium celebrations
- Millennium Dome
- Year 2000 problem
- " Millennium", Oxford Dictionaries (Oxford University Press, 2016).
- "For the Chronologically Correct, Now It's Time for the Millennium". Los Angeles Times. Retrieved April 12, 2021.
- "Millennium: Date Line Politics". WaybackMachine. Archived from the original on June 28, 2006. Retrieved February 6, 2021.
- "When Did the 21st Century Start?". timeanddate.com. Retrieved February 7, 2021.
- Gould, Stephen (1995). Dinosaur in a Haystack. Harmony Books.
- "Millennium Gets Little Notice". The Washington Post. Retrieved February 24, 2021.
- "For the Chronologically Correct, Now It's Time for the Millennium". Los Angeles Times. Retrieved November 13, 2020.
- "Castro hosts party for the 'true Millennium'". The Telegraph. Retrieved November 13, 2020.
- "Japanese purists prepare to welcome new millennium". DeseretNews. Retrieved November 13, 2020.
- Associated Press, "Y2K It Wasn't, but It Was a Party", Los Angeles Times, January 1, 2001.
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