“Blessed and holy are those who have part in the first resurrection. The second death has no power over them, but they will be priests of God and of Christ and will reign with Him for a thousand years.”
—Revelation 20:6.

The “blessed and holy,” those who have accepted Jesus as their Saviour, come up in the “first resurrection.” If they are to “reign with” Christ during the 1,000 years, obviously they must return to life at the beginning of the 1,000 years.

Who are raised in the second resurrection at the close of the thousand-year period?

“The rest of the dead did not come to life until the thousand years were ended.”
—Revelation 20:5.

The “rest of the dead” can refer only to the wicked dead, for the righteous, those saved through their faith in Christ, are brought back to life at the beginning of the 1,000 years.

So the 1,000-year period is marked off by two resurrections: the resurrection of the righteous at its beginning, and the resurrection of the wicked at its close.