Dear Simon ,
Thanks a lot for having invited me to visit your new house this Saturday and I am really glad to go .Unfortunately ,my uncle will be back from America and I have to pick him up in the airport then.I am totally sorry that I can't appreciate your beautiful house this Saturday .But I promise I will find another time to go .
Have a nice weekend!
Yours sincerely,
XXX
純手工標準格式地道表達，希望對妳有所幫助！