Dear Yangling:
Congratulations on your coming birthday.
Thanks for your invitation,but I am sorry that I am afraid I am not able to attend your birthday party.
You know,there is an important English test in our school on the same day.I will have to attend the test .I am terribly sorry for that.
Anyway,I hope everything goes well in your birthday party.Remember to show me the pictures taken in the party.
Best regards!