Hello,
I am so, so sorry, but I just realized I've made a terrible mistake in the deadline for your paper. It's been an extremely busy week for me and for some foolish reason I was under the impression that your document needed to be returned by October 2. It's only this minute that I realized it was supposed to be returned to you on October 1. 
There's obviously no excuse for this mistake, and I feel very ashamed about it, especially after you wrote such a nice comment to me when you asked me to edit your paper for you. 
I sincerely hope that this delay doesn't cause you any problems. I deeply regret any trouble you experience as a result of my carelessness. I will return your document as soon as I possibly can. 
Again, I am sincerely sorry, ashamed, and embarrassed about my error. Your document is my top priority right now.