Albert Barnes on Revelation 20:7

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

Satan shall be loosed out of his prison. See Rev 20:3. That is, a state of things will then occur as if Satan should be for a time let loose again, and should be permitted to go as formerly over the world 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 the part of God. All that is necessary to be supposed 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.