Users are never 100% focused on a task. So it's your responsibility to prevent them from accidental errors, like selecting invalid dates. For example, the system shouldn't allow users to pick the return date earlier than the departure date.

One option to handle this situation is to provide a meaningful validation message indicating an error. A more effective method is to disable invalid and illogical options. It should be visually apparent that specific dates are unavailable. The most popular technique is using subtle grey or making dates inactive when users try to click or tap them.