Revelation 20:7-10 describes it:

“Now as soon as the thousand years have been ended, Satan will be let loose out of his prison,

Satan the Devil and his associate demons have been out of action for the thousand years, unable to interfere with the dispensing of justice and righteousness during that period. Earth’s population has been without such interference from the Devil, the one who, through violent means at his disposal, could keep mankind in general ‘through fear of death subject to slavery all through their lives.’ (Heb. 2:14, 15) Now, for the purposes of the test, Satan will be loosed for “a little while.”​—Rev. 20:1-3.