The story
From the phrase there's no such thing as a stupid question.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: A holiday that is sometimes celebrated, usually by school students and teachers, observed on either September 28th or the last school day of September. The purpose of the holiday is to encourage students to ask more questions in the classroom. Full definition →
From the phrase there's no such thing as a stupid question.
Etymology from Wiktionary, CC BY-SA 4.0.