思い立ったが吉日
Idiom- 1the best time for doing something is as soon as you've thought of it ; there's no time like the present ; it's an auspicious day (for doing it) when you've just had the idea Translated by Jiyomi
Other writings and readings
おもいたったがきちにち
expressions (phrases, clauses, etc.)
JMdict entry 2397940. Source and licence.