Millennium Guide, Meaning , Facts, Information and Description
A millennium is a period of time equal to one thousand years. There are two main viewpoints about naming millennia and there was a popular debate leading up to the celebrations of the year 2000 as to whether 2000 was the true "new millennium".
"01" - "00"
Those holding to 1 AD as the first year of the calendar begin each millennium the year after a year divisible by 1000, e.g. 1001, 2001. The use of the odd year is due to the fact that the Gregorian Calendar has no year zero, and thus the first millennium is years 1 to 1000 and the second millennium starts in 1001.
"00" - "99"
Others argued that the new millennium should begin when the first digit of the year changes; however, as there was no year zero in the Gregorian calendar, this would result in the first millennium being 999 years long. The majority celebrations of the third millennium were at midnight on December 31 1999 / January 1 2000, implying that this millennium will end on December 31 2999. The first decade of the 2000s contains the year 2000.
Others still, such as Douglas Adams, argued that our entire system of dates is so arbitrary and meaningless that it is impossible to define the "true" date of a millennium.
Wikipedia has a page for each millennium: See millennia.