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

"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