Occasion
Buildings hold festivals on special occasions.
A festival can't be happening all the time. Then it wouldn't be special, it would just be a party building, so they can only happen on special occasions.
さい
きわ
None
The occasion this time is to celebrate your wonderful cyborg (さい). It's about time you did something nice for it.
Your cyborg has been really great lately. Your wife loves it. It saved the world. It's nice of you to take this occasion to show the cyborg how much it means to you all with a festival.