[Koha] manage due dates

Hi everybody, I asked this question before (didn't get any answers), but I try again, because I guess everybody must have this problem: how can I enter closing or opening days, i.e. days that should not be a due date? Our library is closed on Fridays and Sundays, on public holidays and for a whole month during the summer. How do I tell Koha to calculate due dates, ignoring the days when the library is closed? I would be very grateful to hear your suggestions. Thanks a lot from Germany! Stefanie Mohsennia www.international-library.de