Isn't "Monday through Friday" intended to be MORE specific/clear than "Monday to Friday"? For example, if I say "The sale runs from Monday to Friday", I may mean that it runs until Friday, so Monday, Tuesday, Wednesday, and Thursday and stops at the beginning of Friday, or I may mean that it is inclusive of Friday and it stops at the end of Friday. If I say "The sale runs from Monday through Friday", it's clear that all five days are included.