Fridays

I have a table that I imported from excel. It has start date, start time, end date end date. I did the datediff function to get the number of hours took, for exampel I wanted a report to show if the start and end date took more than 24 hours, but the problem is that I have some dates that started on friday therefore moset of them too like 72 hours since that saturday and friday shouldn't be counted. Is there a way I could filer for firdays and put a different criteria for them. I want my qry to do the followings:
Give me all items that too more than 24 hours. but if it's friday give me the ones that too 72 hours. Thanks guys.