Capture an invitation as two entries
For a party on June 18 with replies due June 8, the calendar entry belongs on the 18th. Set one person to reply by the 8th. If that person sends a message but receives no confirmation, keep track of which reply they sent. Give the reply task an owner and put the organizer’s actual route for replying in its description. The event itself remains tentative until the household decides to attend.
Check the date labels in context
Read the whole sentence containing each date. “Doors open June 18 at 5” is neither a deadline nor the end of the event. “Book by June 8 to guarantee a seat” changes your decision even if late replies might be possible. Keep that qualification rather than silently turning it into an absolute closing date.
When a reply is ambiguous
If two adults discuss attending, nominate one to answer the organizer and record what they sent. The calendar can say “awaiting confirmation” until the reply is acknowledged if acknowledgement matters. For an invitation that gives no RSVP deadline, ask the organizer or set your own planning reminder and label it as your own.