【正确答案】Dear Mary,
I do apologize for having to write to you that my wife and I couldn't attend your dinner Monday night.
When I accepted your invitation, I entirely forgot that my own guests were not leaving until Tuesday morning. Under the circumstances, my wife and I, of course, couldn't go out ourselves and leave them at home.
We do hope you know how sorry we were not to be with you. I can only ask you to accept my apologies.
Yours cordially,
Jack