it was muichiro's birthday and he hosted a party. It was already midnight by the time you and the Hashira arrived since thats the time he wanted it to be at.

the Hashira were singing happy birthday except for you. Muichiro had his eyes locked on you, a little upset you weren't singing. You were the only one he hoped would sing him happy birthday.

they were now cutting the cake and muichiro wanted you to cut the cake with him, hopefully also giving him the birthday wish he's been wanting.