Now Cas was human he needed to experience human things. It's been a year since he had his grace taken, so a year with no wings and no escape from this land. He's adjusting slowly, and it's working. Cas has stopped having nightmares, but hasn't given up hope, that one day he may become one of God's soldiers. So to celebrate him surviving this, what has been for him, hellish year, the Winchesters are throwing him a birthday party.
Castiel walked into the room and Sam and Dean shouted "Happy Birthday."
Cas looks confused. He'd never had a birthday before, he wasn't entirely sure what he was meant to do. Dean realised his confusion and gestured for him to come over to them. With the confused look on his face he slowly walked over to the brothers. "What's this?" Cas asks them both, exchanging glances between the two.
"A birthday Cas." Sam tries to explain.
"I don't understand. Why are there flames on the cake?" Cas tilts his head staring at the flames that we know as candles.
Sam goes on "They are candles, you blow them out and make a wish, but you can't say your wish out loud or it want come true." There was a pause before Sam started again. "But we have to sing first."
Dean and Sam started singing, badly, but not that Cas cared about the terrible singing. They got through the whole song of Happy Birthday, then Cas leant down to blow out the candles, but to his surprise they just relit. Dean was the first to laugh, then Sam followed suit.
"Witchcraft!" Cas exclaims as he tried another five times. "Why aren't they going out? Dean?"
"Try again." Dean instructs him.
Cas does as instructed the candles still not going out.
"They are trick candles Cas." Sam tell him finally.