Dear Peter,Thanks very much on inviting me to your birthday

Dear Peter,
Thanks very much on inviting me to your birthday party on Sunday. I’d like very much come but I will had an examination on Monday morning. It is a very important exam but I can’t afford to fail it. I’ll spend all the whole weekend reading and prepare for it. So I’m really sorry that I won’t be able to come in this time. Hope you can understand. I’ll take this chance to wish you wonderful time on your birthday. Happy birthday, Peter, and many happy return of the day!
Yours,
Li Ming