後の祭り Ato no matsuri. Too late to be of use.
Traditional Japanese proverb
Authoritative dictionaries define the expression as something done after the proper time, when it is already useless.
Why this date: October 9 is the last day of Nagasaki Kunchi in 2026. The literal 'after the festival' image makes the timing lesson memorable.
Language note: 後 (あと / ato) means after; 祭り (まつり / matsuri) means festival. As an idiom, 後の祭り describes action taken only after it can help.
Good timing is part of good action.
DJW practice example: 締め切りの後では、後の祭りだ。 Shimekiri no ato de wa, ato no matsuri da. After the deadline, it is too late.
Sources: Kotobank — 後の祭り: https://kotobank.jp/word/%E5%BE%8C%E3%81%AE%E7%A5%AD-425922 Nagasaki City Official Tourism — 2026 Nagasaki Kunchi guide: https://www.nagasaki-tabinet.com/blog/tabibu/milk/202609_kunchi-navi