What to do if a guest cancels at the last minute
If a guest cancels at the last minute, tell the caterer as soon as possible to find out whether you can still change the number of covers, and rearrange their table so there isn't a noticeable gap. Whatever can no longer be changed, accept it and move on: last-minute cancellations happen.
The caterer first
Ask your caterer until when the number of covers can be changed and whether there's a charge. Each one has its own terms, and it's best to know them beforehand, not on the day it happens.
Rearrange the table
One empty chair is barely noticed; a half-empty table is. If a whole family drops out, moving someone from another table or merging two small tables may be better. With a digital seating plan it's a matter of dragging people across.
Update the list
Mark the cancellation on the guest list so the head count, the caterer's sheets and the thank-yous still add up.
No drama
Illness, emergencies or work: last-minute cancellations are normal. A kind message to whoever can't come says more about you than any reproach.