I was attempting a Phineas and Ferb quiz but I got one question's answer wrong:
That made me wonder when did that happen? I remember them doing something everyday. What was the day or days when they did nothing?
Season 1, Episode 28.
From the wikia:
Phineas and Ferb decide to do absolutely nothing for the day. Candace questions her purpose in life after realizing that she can't bust her brothers for doing nothing. Meanwhile, Dr. Doofenshmirtz is tired of being the ugliest person around, so he invents a device that can turn anything and anyone ugly.
From the transcript:
Phineas: You know, Ferb, everyday we do something really big. But you know the one thing we haven't done? Relax. I say we take advantage of this perfect day and have the best do-nothing day ever.
Here's a YouTube link, if you want to watch it: