Honestly, he didn't even remeber until Sam spoke up, mid-research, all of a sudden. If he had it his way, Dean was pretty sure he didn't want to remember. He's managed to evade being reminded of the "special" day thrice his whole life and Dean thought he'd pull it off again.
"Hey, almost forgot. Happy Birthday, dude!" Sam said with fake enthusiasm. Or atleast that's what he thought it was.
"What?" Dean had been too busy to even notice the fact that it was his birthday. The pounding inside his head, courtesy of a certain douche angel, had him distracted enough already.
"I said, "Happy Bir-" Dean glared at Sam, raising his eyes from the spot at the table which she had been staring at for a while.
"Why?"
"What do you mean, 'why'? It's your birthday, your day."
"Sam, we haven't celebrated my birthday in forever." It was true. The past couple of years, everything had been too much of a blur to do anything.
"Exactly. And somehow, we always get to celebrate mine. Which is why.." His brother fot out of the chair he had been sitting in and went to the kitchen. When he returned, there was a tray in his hands.
"Is that..?" Dean never got to finish.
"Pie?" Sam grinned. "Yeah, I made it myself," Sam proclaimed, setting down the tray and the two spoons and forks inside. It didn't look good, of he was honest, Sam's pie looked barely edible. And it had a scrawly 'Happy Birthday Dean' written on it.
"Well, what can I say Sammy? Looks like you killed the poor thing."
"Jack helped me finish it off."
"'Course he did. And lemme guess, you hhad this planned for a couple days?"
"A couple weeks, actually but who's counting?" Dean didn't know what to say. It was silent for a while and then Sam opened his mouth, took in a breath, then closed it like he wanted to say something but decided against.
"What?" Dean questioned.
"You're," Sam's lips quirked up, trying for a smile. He whispered "Forty."
Dean contemplated that. It didn't matter to him. The only hing that was surprising was he fact that he managed to somehow live this long. "So?"
" 'So?' Dean, you are officially in the middle age category. You're old."
"Ha ha. Laugh all you want. It'll be your turn in a few years, Sammy." They fell into silence again after that. Sam probably lost in his nerdy thoughts and Dean trying to fend off Michael. Who was currently speaking to him in his mind.
If he lives that long. If you live that long.He had to show him, then. He had to keep Sam safe. He vowed to himself then and there that Dean would try his best to see forty-one. For Sam. And maybe just a little bit to prove Michael wrong.