1630s, "the 1,000-year period of Christ's anticipated rule on Earth" (Revelation xx.1-5); from Modern Latin millennium, from Latin mille "thousand" (see million) + annus "year" (see annual); formed on analogy of biennium, triennium, etc. For vowel change, see biennial. General (non-theological) sense of "an aggregate of 1,000 years, a period or interval of 1,000 years" is attested by 1711. Meaning "the year 2000 A.D." is attested by 1970.
[T]he men of the modern world—up to a generation ago anyway—saw 2000 as a millennial year in the light of science. Men were then to be freed of want, misery, and disease; reason and advanced technology would rule; all would finally be for the best in what would then be the undoubted best of all possible worlds. [Bulletin of the Atomic Scientists, December 1962]