Dear Peter:
Hi!i am Liming.I am very happy when i received your invitation,but i have to tell you that i can not go to your party because Saturday is my grandma`s birthday too,i had promised my grandma i will go to her house and held a party for her.So,i am sorry,i can not go to your birthday party,and thanks for your invitation.Don`t worry,i will send you a gift on saturday,and i will miss you every day and every night.
Yours sincerely
Liming