I know why people would prefer Thursday testing but Monday makes more sense. It means teams can relax and go through a test regeime without worrying about a race and not bending anything. It also means that they can use it to actually test against some live race data rather than just a glorified set-up session.
If you have it on Thursday, it would be more about setting the car up for the weekend rather than allowing a 3rd driver to test and possibly gain experience.
