How to require a student to pick 2 days out of 7 before continuing

I created a calendar with checkboxes on it.  I want my online students to check AT LEAST 2 boxes before the "next" button appears (they have to choose at least two days to "come to class").  If they only click one button, I want a layer to chow telling them to pick at least two.  I am attaching a screenshot to explain.  I have been trying to add variables, and it looks as though I have to do 21 pairs (M+T OR M+W OR M+Th, etc. for all combos of at least two).  However (1) I can't seem to nest the variables - (M+T) OR (M+W) OR (M+Th) etc., and (2) even if I could, it looks so messy.  Is there a shorter, simpler way to require a student to pick at least 2 items before going on?

Thanks--

--Susan