Sally,
This is a quick note to tell you that I can't see you on Saturday.
I'm sorry, but Paul (my brother) is coming home on/at the weekend and in the evening we're going to a concert. But I'm free next weekend, if that's okay with you. We could meet on Saturday afternoon, go shopping, and then go out to dinner.
Remember, it's my birthday on June 15. I got promoted  yesterday. I start my new job at the beginning of July and on July 7 I fly to the U.S. I'm kind of nervous about this at the moment.
Brian