【正确答案】
Dear Eli,
I am writing to tell you that I won't be able to attend the wedding ceremony of your daughter.
Last week I received your invitation and knew that the wedding ceremony will be held at 3:00 pm this Saturday. I hope to attend it very much, but I can't because I have already promised my sister to help her move into her new house this coming Saturday and Sunday. You know that there is always much to do and much to be taken care of when people are moving into their new house. However, I am going to pay a special visit to the new couple next weekend. Please pass my best wishes to your daughter and your son-in-law.
Accept my sincere apology again.