My dear Miss Haines,
We regret exceedingly that we cannot accept your father's kind invitation to dine with you on Wednesday, January the fifteenth. A previous engagement of Mr. Evans prevents it. Will you convey to him our thanks?
Very sincerely yours,
Katherine Gerard Evans.
January 5, 1922.
Adding additional details
The invitation to an informal dinner may necessarily include some additional details. For example:
Five Hundred Park Avenue,
September 16, 1920.