I own nothing, though I would like a Cas! Done for an April Fools' challenge.

Sam could hear Dean singing in the shower as Cas stood opposite him.
"Look, it's just this thing people do. It's fun, it's meant to be a joke. Please will you help me?" he asked imploringly.
"I do not understand why you do this, but if it is truly human tradition I will participate."
"You're brilliant," Sam replied as he beckoned Cas closer. "Here, I've got a great idea..."

Dean towelled his hair dry and got into his clothes cheerfully.
"You guys ready for breakfast?" he asked with a smile on his face. "I'm super hungry."
"Yeah, we'll be out in a minute. Just getting packed up," Sam replied, keeping an expertly straight face.
"Ok, whatever. I'll get the car going," came the reply.
The door closed behind them. Sam waited expectantly, Cas watching Sam trying to understand this bizarre human ritual. After a moment Sam got his response and a huge grin broke out across his face.
"SaaaaAAMMMM!" came a panicked semi-scream from outside.
Sam just about managed to regain his composure as the door burst open.
"Sam! The Impala! It's, it's gone, Sam! The Impala is gone!" Dean's face was pale, and Sam could swear he saw sweat on his brow.
"What? No way!" Sam feigned in response. He quickly raised from the bed and ran through the door. He took a moment to scan the car park before his panicked face softened. He turned to Dean.
"Look, the car is over there," he pointed. "Same reg plate as usual, don't you remember? We're fine, it's all good."
Dean's face didn't change, if anything it became even more scared and his voice rose higher, "That, Sam? That's a pink Prius! You think I'd know if our car was a freakin' PINK PRIUS! Someone's stolen the Impala, Sam! It's got EVERYTHING in it! My baby..." his voice trailed off.
Sam beckoned to Cas behind Dean's back, a smile cracking his face for a moment while his big brother wasn't watching. Cas walked over the boys. Dean turned to him, with what looked like tears in his eyes.
"Cas, can you get the Impala back? Can you find it?" he begged, desperate.
"I don't know what you mean, Dean. That is the car we have been using since I have known you."
Dean looked back and forth between the angel and his brother. Could he be going crazy? He pulled out his car keys. They were pink. And for a Prius. If this is that Trickster ass again I'm going to kill him.

Dean's knuckles were white as he gripped the steering wheel. Sam looked at him with a feigned concern as Cas sat, expressionless, in the back seat. The silence was getting more and more awkward, and Sam had honestly expected Dean to have got it by now, but he didn't seem to keep track of dates anymore.
"Hey Dean, how long is it until we're gonna get to Bobby's again?"
"Two days."
"April third then?"
"Yeah."
Dean's words were as short and stiff as possible, clearly he was upset.

Ten minutes went past.
"Wait," came Dean's voice. The car started to slow down. "April third."
"Yeah," Sam replied. Looks like he's got it! He steeled himself for the resulting outburst.
"Which means today is April first."
"Yup," replied Sam, trying not to give away any emotion.
Dean was silent for a moment.
"You're such a dick Sam."
Cas looked at Sam, a confused expression on his face, "Sam, I thought you said this was human tradition? Why have a tradition which promotes anger?"
Sam slowly closed his eyes and sighed. Here we go.
Dean stopped the car and turned to look at Cas, who seemed to cower in the intensity of his gaze.
"You did this? What the hell Cas! This is screwed up, you don't mess with people like that!"
Sam placed a hand on Dean's arm, "Hey, it's not his fault. I told him to, you gotta admit, this is the best prank either of us have pulled."
Dean looked at him. "You do realise what you've started, right?" He turned back to Cas. "You owe me one Cas, and you better help me out when I ask."
"Now," he addressed both passengers, "get this... thing turned back into the Impala. Now."