Often, users would go on holiday or may not be able to action tasks on a specific date. Currently, there is no feature in the workspace to prevent a user from being assigned a task on a given day. Other members of the workspace may not be aware that the user is on holiday etc and would continue assigning tasks to a user that would not be actioned until they returned.
We're currently doing a major rewrite of Tasks and Calendar features. We'll consider how/if we might be able to implement something like this later on in this development project.