後の祭り (あとのまつり, ato.no.matsuri) is an idiomatic expression that literally means ‘post-festival’. Figuratively, it indicates that it is no use regretting the past; you can’t enjoy food stalls and live music anymore after a festival is over*. For instance, if you regret not studying hard after the exam is over, that’s 後の祭り. It is also used to describe when an action or event is too late. For instance, it is 後の祭り even if you say to your partner a million times, ‘Sincerely I love you.’ after you ‘accidentally’ called him/her by the name of your ex.
* In fact, there are a few other theories in its etymology. One of them claims that 後の祭りwould refer to the second-half period of Kyoto Gion Matsuri, one of the largest festivals in Japan that lasts for about one month. The festival is usually more boisterous during the first-half period than during the latter, and the idiom may refer to the idea that it’s no use or too late to attend the festival after the main part is over.
Similar Phrases in English
In English, idiomatic expressions similar to 後の祭り include, ‘the ship has sailed’, ‘shut the stable door after the horse has bolted’ and ‘cry over spilled milk’. The closest English phrases are ‘(something is) too late’ or ‘no use regretting about something in the past’.
Even if you say you should’ve confessed your feelings at that time, that’s ‘post-festival’ (it is no use regretting about it).
cf. the post about 告白する or 告る (Kokuru): Japanese Slang ‘Confess Your Feelings to Your Crush’
Even if you’ve just noticed he was interested in you, that’s ‘post-festival’ (too late), because he has a girlfriend now.
Synonyms 手遅れ (ておくれ), 後悔先に立たず (こうかいさきにたたず)、覆水盆に返らず (ふくすいぼんにかえらず)
Synonyms in English: too late, no use regretting the past
Related Word: 後 (あと) post, 祭(まつ)り festival