因未能践约赴宴致歉 Unable to keep one's promise
Dear Miss Nancy,
Much to my regret I was unable to keep my promise to attend your birthday party last Saturday, owing to the fact that my little son was suddenly taken ill early that day.
Hoping to see you soon.
Truly yours,
Tom
亲爱的南希小姐：
小儿上周六突然生病，因此未能应约赴生日宴，殊觉抱歉，希原谅，再见。