Dear Miss Nancy,
Much to my regret I was unable to keep my promise to attend your birthday party last Saturday, owing to the fact that my little son was suddenly taken ill early that day.
Hoping to see you soon.
This is to let you know that I can't go for our appointment as the time is overlapped with my course schedule. I do apologize for it. We can find another time to meet at your convenience. Just let me know when it's suitable for you.
I'm sorry to say that I cannot go to the bookstore with you on Friday afternoon. I have just found that I have to attend an important class meeting that afternoon. I hope the change will not cause you too much trouble.
Shall we go on Saturday morning? z.xxkWe can set out early so that we'll have more time to read and select books. If it's convenient for you, let's meet at 8:30 outside the school gate. z.xxk If not, let me know what time suits you best. I should be available any time after school next week.