The fact that Graves makes Cronus (aka Saturn in the Roman pantheon) titan of Saturday and comments that Hermes is the “god” of the fourth day, suggests that he is using the traditional Western etymology for days of the week. That means;
we accept Nagy’s notion that the name of a son can reflect a main characteristic
of the father; (CHS Open House discussion) maybe we can make a few more assumptions on
the ancient Titans.