One possible version:
Dear Dr. Smith,
How are you doing recently?
As you are going to leave China, we intend to hold a farewell party for you on Friday, October 27th at Room 1002, Building 8. I' m writing to invite you to join the party. It will begin at 7:00 p. m. and last two hours. On that evening, all the English teachers and the students in your class will be present. We have prepared various interesting programs for you, including songs and dances. And we' re very excited to hear your speech then. Hope you can join us and have some fun.
Looking forward to your early reply.
Yours,
Li Hua