There *has* to be some sort of formula or method when it comes to scheduling the TSVs.
It's no coincidence that big ticket investment/"family" items like mattresses, computers, vacuums, cameras, etc. are on the weekend, and when they're on, they present them even more frequently than a TSV during the week (IMO). QVC also has two big shows to work with on the weekend, 10-2pm Saturday and David's show on Sunday.
Notice that David almost always presents the TSV after his ITKWD show on Sundays. So, he's not going to be presenting an IT Cosmetics makeup kit, LOL. Could he sell it? Well, yeah, but they need him selling the big ticket items. Plus, if I'm not mistaken, David sells quite a bit in general. I thought I read somewhere that he is the top-selling host.
Beauty and fashion TSVs are mostly on Mondays and Fridays, I think. They also have kitchen and food related TSVs on Wednesdays. (Again, so David can present them.)