Dear XXX,(道歉并感谢) Thank you for inviting me to your house for a dinner.I'm really appricate for that and would like to go.But I have a date on that day.So I'm sorry that I can join you.Thanks again for your hospitable inviting. Best regards!Yours Sincerely, XXXXX Dear XXXX,（感谢信—表示可以参加） I have received your invitation letter.Thank you for inviting me to have the dinner in your house.I'm glad that you have still thought of me for a long time.And don't worry and I'll go there in time. Thanks so much again. Best wishes. Yours Sincerely XXXXX
