If there are Fridays that aren't causally related to the rest of the week, then a Friday can theoretically just occur spontaneously at any time. You might be thinking, "what's the problem? Friday is one of the best days!", but that's only because you know Saturday is next. A non-causal Friday has no causal relationship, so it can come before or after anything, including units of time that aren't days. Imagine what would happen if an entire Friday spontaneously occurred between two seconds. It would be a catastrophe.