Dear Zhang,
I am really sorry to tell you that I was unable to attend your birthday party yesterday evening. That is owing to the fact that my old friends visited my house without telling me the exact time. I had to entertain those friends with my passion in the entire evening,and I will continue to entertain them in the following two days.
I really regret that I haven't go to celebrate your birthday personally and had missed a wonderful chance to enjoy myself with our kind colleages. I have prepared a pretty gift to you yesterday and will send to you this afternoon to show my best birthday wishes. Besides, I trust that you will understand me.
Best wishes,
yours sincerely,
Tom