Dear Mary
I have received your invitation. I'd love to take part in your birthday Party, but my Saturday has been occupied. I am sorry to say i will not come to your party. In the morning on Saturday I will attend the English competition. And in the afternoon on Saturday I have to go to the football club to play football. In the evening my grandpa will ask me to go to his house to have dinner with him.
Happy Birthday to you!
Yours friendly
Tom
(除去开头和结尾,大概70字左右.）