There is a very new feature in request form that allows creators to set constraints that only allow the entry of dates that are X or more days later than the form submission date. In practical terms, this means when someone fills out a form, they won't be able to pick dates (for example, a due date) that are earlier than what the form creator has designated.
HOWEVER this does not include blocking out weekends which means that if you are trying to fix a SLA say for 5 days it includes the weekend and gives the requestor the illusion that something can be delivered soon than possible.
I suggest you allow each user to set their own weekend and block out days - and add this feature to the Constraints for Date question.