Answer from Kate, SHRM-CP:
You can generally ask employees to provide more information when they indicate that their vacation or PTO request is for personal reasons, but we don’t recommend it. The specific reasons for the day off shouldn’t affect whether you grant the request, and you don’t want to give the impression that you’re accepting or denying requests based on the reasons that are given. It may also be information an employee doesn’t want to share.
If the time off requested is problematic for scheduling and you’re trying to assess whether the employee could take off a different day or time (or if the company should grant the request despite the inconvenience), we recommend having a conversation with the employee rather than digging for details without telling them why you’re asking.
This advice applies only to time off that you offer as a company benefit. Time off that is required by federal, state, or local law will have its own requirements about what you can or cannot ask.