有英文請教

Dear Sir / Madam,
Thank you for your kind invitation to join the farewell party in our company next Saturday. I regret, however, that I shall not be able to come because I have already made arrangements to spend that weekend in Macau with visiting relatives from America.

I understand your are inviting quite a number of people, so I trust my absence will not upset your plans. Let me apologise once again for not being able to come. I assure you that had it not being for my relatives' visit, I would have accepted the kind invitation with which you had honoured me. I look forward to another occasion when we can get together and have a pleasant time as we have had in the past.
Yours faithfully,
Donald.