Dear ______,
Much to my regret / I am terribly sorry that I am unable to ______ . It is mainly because at that time I will be full occupied by which is quite out of my expectation, and that conflicts with your . Therefore I am forced to adjust my schedule and cannot make my presence on your . I sincerely hope that you will enjoy a good time. By the way, can we have dinner together some time next week? I hope to see you soon. I am really sorry again and I trust that you will understand.
Yours sincerely,