Dear XXX,
I am so sorry to tell you that I have to XXXXX on XXXday so that I can't be in XXX that day.
Mr. Lee told me this morning that he will help me solve the problems I asked him on the last homework. Also, that day is the only day he have time to help me before the next English test. So I think I would better to meet him that day.
Please understand me and I will invite you next time.
Sincerely,
XXXX