Dear Tom,
It's a long time since we met last time. I miss you very much. How’s everything going? I’m writing to say sorry to you because I can't obey our promise. Don’t be angry. Please give me one minute to explain it.
In fact, my mother asked me to visit my grandma that day. You know grandma is to me what water is to fish. I had a good memory with her. And we should spare no effort to take care of the old, shouldn't we?
Friend, you're still young but my grandma is old! Taking these factors into consideration, I decided to write the letter finally.
Let's make it next Saturday. I think you can understand me. Now, say sorry again, looking forward to your reply.