Overlapping of an event…how do I fix this?

Ok, so I'm trying to put my class schedule in my calendar. I need them to recur MWF or TR. Also, I was trying to make it where they will not recur after a certain date when that semester ends but whenever I try to do that, I get a little message saying: "Occurrences of this event are overlapping. If you don't want this, change how often it recurs or how long it lasts."