The Grim Adventures of Billy and Mandy S01 E14 A Grim Surprise

"It's Good Grim Fun!"

Billy tells Grim to help him throw a surprise birthday party for Mandy. Then, a reluctant Grim helps Billy prepare a cake while keeping it a secret from Mandy. In the end, a somewhat surprised Mandy reveals that her birthday isn't for the next five months, something that Billy already knew about.