Dear [Wang Hua]:
I'll be delighted to come to your luncheon on [Tuesday, April the sixth], at [on] o'clock.
[Bob] has often spoken me of [Wang Hui], and has told me how very much he enjoys having him for a [roommate /teammate]. Although I have never met [Wang Hui], I know him from hearing so much about him.
I assure you it will be a very great pleasure indeed to meet [Wang Hui's mother]! Thank you so much for asking me.
Truly Yours