Albert Barnes Commentary Revelation 20:7

"And when the thousand years are finished, Satan shall be loosed out of his prison," — Revelation 20:7 (ASV)

And when the thousand years are expired. See Revelation 20:2.

Satan shall be loosed out of his prison. See Revelation 20:3. That is, a state of things will then occur as if Satan should be let loose again for a time, and permitted to go about the world as he did previously; no intimation is given why or how he would be thus released from his prison. We are not, however, to infer that it would be a mere arbitrary act on God's part. All that is necessary to suppose is that there would be, in certain parts of the world, a temporary outbreak of wickedness, as if Satan were for a time released from his chains.