Create a multiple day service that must start on a Monday

Hello, 

I am offering a service that lasts 10 working days, but I want the 10 days to start on a Monday. 

I have created a service that lasts 10 working days, but when you book it, you can start the booking on any day of the week and it goes forward 10 working days from there.

How can I limit this service so it is only bookable on Monday?