Thank you for contacting us. Unfortunately, there is no direct way to do this, but you could modify your search query and instead of searching for "7 days" you can search for "mon, tue, wed, thu, fri".
We will consider adding a better way to do this and you're welcome to add a request for such feature to our Votebox: http://todoist.com/Vote/
This also works (if you are premium) and you create a custom filter :
overdue, 7 days & !sat
Show overdue and the next 7 days except sat.
By a strange quirk (bug?), it seems that when you put the '!sat' it exclude sat (whether there are tasks on that day or not - as expected) BUT also any other empty day (including Sun if that is empty - or just add '& !sun' to the above to always exclude it).