Hello,
There is possible settings in Configuration -> Calendars -> Nights style
I am not sure it is what you want. This functionality works this way:
1. If you select Monday to Friday, it is for 5 nights (6 days) start on Monday and ends on Saturday.
or
2. If you select Mon/Tue to Fri/Sat works same way but shows you that it ends in Saturday.
If I understand you right, you want this functionality:
Someone select Monday to Friday and it means reservation for 4 nights (5 days).
This is not possible in our actual system with night booking.