思い立ったが吉日

おもいたったがきちじつ, おもいたったがきちにちomoitattagakichijitsu

expression

1. the best time for doing something is as soon as you've thought of it
2. there's no time like the present
3. it's an auspicious day (for doing it) when you've just had the idea

Study it in Yanamino