I had to edit a bit because I made a mistake
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Everyone can guess that Dabi hated parties and wouldn't want one for his birthday.
I'm pretty sure he hates his birthday too but he'll endure it for your sake.
So he would just get annoyed if you did a birthday party or/and get him a present.
So if he really did want anything for his birthday it would be.....
No not laying down and cuddling
It would be....
✨arson✨
Especially committing arson with you helping him.
So what did you do for his birthday?
You got him a whole abandoned town made out of wood that could easily inflate and cause destruction
(Is it weird I find it satisfying seeing wood from a house slowly burn and fall off the building? Or anything slowly burn really)
He was happy as you spread gasoline everywhere and he burned the whole town down slowly watching the town burn and fall apart.
And then you two made s'mores and cuddled together until the police came from the report of a wild fire.
And as you all guessed
He burned the cops and carried you away.
It would be romantic if he didn't leave a bunch of burnt bodies in his trail