Reserve only the part of the day that matters
Ask what taking the hold would prevent someone else from doing. Jo may visit for two hours, so Saturday from two to four might be enough. If Jo needs the whole afternoon but has no arrival time yet, say exactly that. Include who is checking with Jo. The hold should contain the uncertain fact, who can resolve it, and the next time someone will look for an answer.
Give the hold a real end
Say what happens at the deadline. For a guest who has not replied, you might release the afternoon and offer another date. Tell anyone who was holding the time that they can make other plans. Removing the calendar entry alone may leave them waiting for a visit.
Handle a competing invitation while the hold is open
Suppose a friend offers tickets for the same afternoon on Wednesday. Check when the friend needs an answer. You can ask Jo whether the visit can be confirmed early or moved to Sunday. If neither is possible, decide with the people holding the time whether to keep waiting. When the hold becomes firm, change its label and add the place. When it expires, free the time before making a new commitment.