This is a sample apology letter to a friend for not keeping your promise.
Dear …………………… (Write your friend name here),
I am very sorry. According to our programme I was prepared to join you in the picnic. But the evil hour came at last. I could not keep up your promise. You must have felt disappointed for my failure to join you. My dear friend, I hope, you will not misunderstand me. I am sure, you must realize my situation. I was completely undone.
Two days ago, mother suddenly broke down in the morning. I was reading in drawing room at that time. As you know she was in poor health. I was much frightened when I saw her In this state. I sent for our family doctor who made every effort to restore her to consciousness. After two hours of anxiety, she could open her eyes and reply to our eager questions. The doctor wrote some medicines and asked me to take care of the patient “with proper dose of medicines at right interval. So I was to attend her for the whole day. My father was absent. He had been to my uncle’s house that day. He came back the next day.
However, she could not be strong in her feet for three days. She is feeling better now.
I think, you are now able to realise why I could not keep my word. You will judge me rightly and forgive me for my failure in joining the proposed picnic.
……………………………… (Your Name Here)