Meaning of 後の祭り in Japanese

Definition of 後の祭り

あとのまつり(atonomatsuri) 【後の祭り ·後の祭】

(n) Too late!
いまさら嘆いても、それこそ、後の祭だよ。
You can regret it all you want, but it won't do you any good now.

今更いまさら悔やんでも後の祭りあとのまつりだ。

It is too late to repent.

With 20/20 hindsight, he could tell why they lost the game.

Yelling about it won't help now. You should have done that when it counted.