Dear xxxx:
I am having a birthday party this saturday and would like to invite you to come along.I am turning eighteen,which is a big turning point in my life and would want all my friends and family to be there to share this special moment.It would be a casual barbecue at a park near my house and I will send you the exact address closer to the date.Please contact me as soon as possible to inform me of your attendance.
RSVP:before friday
xxxx
Dear xxxx,
Thank you so much for your invitation to your birthday party this Sunday evening.I'd love to come,but I am afraid I cannot make it.I will leave for Beijing on a business trip the day after tomorrow.And I will have to stay in Beijing for several days.So I am afraid I will not be able to come back until next Monday.Please say hi to all our friends.I wish you all a good time!
Happy Birthday!
All the best,
xxxx