The Millennium is ushered in by Jesus’ second coming.
But to be honest with you, there are two other views of when the Millennium occurs. There is a postmillennial view that teaches the Millenium is literal, is ushered in by the church, and Jesus comes after the thousand years. And there is the amillennial view which teaches the Millenium is symbolic rather than literal, that the Millennium refers to the time between Jesus’ ascension and his second coming, and the Millenium ends with the second coming of Jesus.
But I don’t think the Bible gives good evidence for them, so I teach what is called the premillennial view. I believe the Millennium is a literal 1,000 year period of peace for which Jesus will set up his earthly kingdom. I believe it is ushered in by Jesus’ second coming, at the last conflict of Armageddon, when the Antichrist and the False Prophet are cast into the lake of fire, and their armies have been destroyed. So I believe the Millennium begins at Revelation 20:1. For me, that is how John lays it out. I don’t see the Bible teaching a postmillennial or amillennial view.
Is there room for disagreement? Absolutely. This is not an essential doctrine. What I mean by that is that our view on when Jesus comes again, when the Millennium will be, and if it is literal or symbolic is not something that will keep us out of Heaven. I don’t think these different views should cause division or controversy. However, whatever you view you take, you should believe because that’s what you have concluded the Bible teaches. As for me, I believe the Bible teaches a premillennial view.