I am very sorry that I didn't attend your birthday party last Sunday. I feel awful about it and want to apologize to you.
It was nice of you to invite me to your party. I should have come and celebrated the precious moment with you, but the monthly test is around the corner, so I was so busy preparing for the test that I forgot the appointment. I hope you understand my situation and forgive me. Is it possible for you and me to have a private meeting after the test? I do long for a pleasant chat with you.
It is much to my regret that I missed the chance of a happy get-together. Once again, please accept my sincere apology.