Why is the millennium a seventh period of testing?
The millennium is the seventh period of testing because six previous periods of testing have already occurred throughout human history, and God always does things in seven. At the end of the thousand years, Satan is loosed for a little season to test the new generations arising in the kingdom economy, making this the seventh testing of the human race.
Les explains that Satan is confined to the bottomless pit for the thousand years because the Lake of Fire is eternal, and Satan is not ready for eternity. He has one more opportunity at the end of the thousand years, when he is released and given full freedom to deceive the nations. This release allows him to test the new generations that come on the scene in the kingdom economy, so the millennium becomes another period of testing. Since there have already been six periods of testing throughout human history, this one must be the seventh, and God always does things in seven, requiring a seventh period of testing the human race.