YOU CAN’T POSTPONE...have you ever organized an event? Once you’re gone...others come in...it’s over...
How do you know the venue is already booked and they can’t have it for Fri?
I was on the planning committee of a much smaller convention in 1994. It took over a year to make arrangements for the hotel, food vendors, suppliers, entertainment,... for that one. It's definitely not something that you can just find a substitute for in a week or two.
Plus at this stage, the attendees have already arranged time off from work, made their hotel reservations, bought airline tickets. So you can't just tell them the day before the event to try to come back in a week or so.