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I think you'll understand everything if I tell you this: your weeks, considering every other one, form a cyclic group of order 2, isomorphic to the group of residues modulo two.
In short, if the remainder when dividing the week number by two is two, and the day of the week is Monday, then....
if (( GetDate(WEEK) == 2 ) or ( GetDate(WEEK) == 4 )) and ( GetDate(DAY_OF_WEEK) == 1 ) thenAnd now everything works as it should.