Millennium is a period of one thousand years or a thousandth anniversary. A millennium is a period of time, equal to one thousand years. The term millennium may implicitly refer to calendar millennia; periods tied numerically to a particular dating system.
It has been argued that the arrival of new millennium should be celebrated in the transition from 2000 to 2001 (i.e. December 31, 2000). That since the Gregorian Calendar has no year zero, the millennia should be counted from A.D. 1. Thus the first millennium of one thousand complete years runs from the beginning of A.D. 1 to the end of A.D. 1000, and the beginning of the second millennium took place at the beginning of 1001. The second millennium thus ends at the end of the year 2000.
In Christian tradition the Millennium refers to the one thousands years of peace that will accompany the second coming of Christ. Christ's coming is portended by growing strife and conflict (an apocalyptic vision). Millenarianism, refers to the belief in the doctrine of the Millennium (the coming of Christ).
The non-religious beliefs of Millenarianism have been associated with the end of one hundred year periods (the end of the 18th century or the 19th century). Historically, there has been debate around the turn of previous decades, centuries, and millennia.
There are two viewpoints about how millennia should be thought of in practice, one which relies on the formal operation of the calendar and one which appeals to other notions that attract popular sentiment.
There was a popular debate leading up to the celebrations of the year 2000 as to whether the beginning of that year should be understood (and celebrated) as the beginning of a new millennium.