I'm starting to find that my best days for converting is a Tuesday and worst is a Saturday. I'm not convinced enough to go into ad scheduling, but it's always interesting to see that some days consistently poor and others are great. Of course it will differ depending on your account purpose. Mine is offering a business opportunity so perhaps Tuesdays is best as people are finding my website when sat at work bored out their skull and want to get out of the job they are in? Maybe Saturday's is the worst because people are off work and hungover so don't have the energy to read the site
My best converting days is in weekend, and poorly convert when Monday until Friday. My site about health treatment guides.
it really depends on what niche you are in... If you are looking for a universally bad day I don't think it exists. Every day of the week is really good for some and really bad for others.