Ash had lied to the others.. Wanting attention.. She told them, her birthday, was on the exact days of yours (3rd of December), which they didn't remember, their to busy with Ash.. So your birthday, is saved as hers for them.. And when you wake up, you go into the kitchen, get a monster, and see them giving Ash a dream birthday.. She even got a sweater (Reference for the song - "Heather") from Ghost..