An idiom meaning it's too late to do anything; a case of 'closing the stable door after the horse has bolted.'
連絡が遅すぎて後の祭りだった。
The contact was too late; it was already too late to do anything.
謝っても後の祭りだ。もう手遅れだ。
Even if you apologize, it's too late. It's already past the point of help.
予防を怠ってしまい、問題が起きてからでは後の祭りだ。
We neglected prevention, and once the problem occurred it was too late.