Scary Birthday to You! is the 39th book in the Give Yourself Goosebumps series.
Your parents have to go to work on your birthday. They hire a strange entertainer called Dr. MacDeath to throw you a horror-themed birthday party, but he soon proves himself to be evil, and turns one of your friends into a balloon animal. He nails shut all the doors in your house so you can't escape. After an interruption from someone at the door, he forces you to cut your birthday cake, which turns out to be crawling with bugs and worms. You must then decide whether to play party games or open presents.
You play party games, with a choice between Paintball, Dodgeball, Pin the Tail on the Donkey, or a Scavenger Hunt. You have to survive the games and find a way to beat Dr. MacDeath, but he's not exactly playing fair.
You decide to open presents. Your friends' presents have been enchanted by Dr. MacDeath, but he has a few surprises of his own for you.
Side story C
In story B, you open a present Dr. MacDeath gave you, which contains two tickets to a horror-themed amusement park. You take your friend Josh with you and can have fun without MacDeath, but this just leads to two possible endings (one good, one bad.)