Setsubun is a Japanese holiday celebrating the beginning of spring. To know more, look it up in a search engine.

"Oni wa soto! Fuku wa uchi!" Was all the warning he had before several small objects struck him. Heiji didn't so much see as feel himself transform into his oni form.

"Ack!" He glanced at his now dusky-red skin, then up at Kazuha who was grinning like a cat in the cream. "Kazuha! What'd ya do that for?!"

"Well, today's the day for driving evil spirits out of the house, so..." She mischievously flicked another soybean at him. "Out, Oni!"

He raised an eyebrow. "Y'know, if ya wanted me out of the house to go on a date..."

"HEIJI!"

-fin-

"Oni wa soto! Fuku wa uchi!" = "Devils/Oni out, Happiness in"
It's shouted while throwing soybeans at oni.