Dear Ms. Ann,
I have received your invitation through your friend. It was nice that someone like you would invite me to be a guest on your special event.
I am accepting the invitation with the gratitude and honor. I have read the details of the event and I have included it on my schedule for that day. Rest assured that i will be present on time. I have already made a reservation for the trip and I am waiting for the time with glee. I know I will be having a good time with you, as I always did during the times that we were together in the past.
Do not worry over anything more that you could do for me during the event. Being invited was enough for me. Thank you. I am looking further to be with you again.