後の祭り/the opportunity has passed; regret something as too late

祭りが終わったあとの山車や祭祀用具の意から、時機が過ぎて役に立たないことのたとえ。また、後悔しても手遅れのことをいう。

Comes from the idea of the float and ritual items being left over after the festival. An example of the opportunity having passed and things that are not useful being left over. Regret at being too late.

山車・だし・float
祭祀・さいし・rite, ritual
時機・じき・chance, time of an opportunity
後悔・こうかい・regret
手遅れ・ておくれ・too late