A proverb meaning that the best time to do something is the day you decide to do it — don't delay; act at once.
思い立ったが吉日、今日中に申し込みを済ませよう。
There's no time like the present — let's finish the application today.
ずっと習いたいと思っていたピアノ、思い立ったが吉日で体験レッスンに申し込んだ。
I've wanted to learn piano for a long time; since I decided to, I signed up for a trial lesson right away.
『思い立ったが吉日』という言葉の通り、まず一歩を踏み出すことが大切だ。
As the saying 'there's no time like the present' goes, it's important to take the first step.