It was Christmas Eve, and all the hashiras, including you, were at Master Kagaya's Mansion to celebrate for it

There was a mistletoe under the door frame, and you were standing below it, unaware that it was there, then Muichiro came under it as well and kissed you on the lips

Muichiro: "That's what happens when you stand under a mistletoe y'know~" he said with a smirk on his face and holding your chin with the tip of his finger, clearly proud of what he did