Ask for two explicit alternatives
Reply “Do you mean Friday the 12th or Friday the 19th?” Keep the answer with the event details. If you must make a temporary calendar entry before getting a reply, label it tentative and give someone a task to confirm the date. Include the month and year if the conversation crosses a month or holiday. Exact dates also help people reading a message later in another time zone.
Check the event rather than the forwarding time
An invitation sent on Monday can be forwarded on Thursday, then read the following week. Record when the organizer actually wrote the relative phrase if you can identify it. Do not resolve the date from the time you received a screenshot from a friend. The safest next step is a short reply to the organizer.
Handle a tentative hold
If either Friday would create a real conflict, tell the household that two dates are possible and identify who will confirm. Release the unused hold once the answer arrives. If one date has already passed, that narrows the possibilities but does not prove that the later date is correct.