Final answer:
To ensure that event details in a shared calendar remain hidden, Dan should use the private command.
Step-by-step explanation:
To ensure that delegates and users of his shared calendar cannot see the details of a particular event, Dan should use the private command. This command allows him to mark the event as private, which restricts the visibility of the event's details to only Dan and those he explicitly gives permission to. By using the private command, Dan can prevent unauthorized access to sensitive information.