April Fools' Day
From Longman Dictionary of Contemporary EnglishApril Fools' DayˌApril ˈFools’ Day (also All Fools’ Day) noun TRICK/DECEIVEApril 1st, a day when people play tricks on each otherExamples from the CorpusApril Fools' Day• At first I thought it must be April Fools' Day.• He was indicted April Fools' Day.• He went to police college on April Fools' day, 1970, where he was the best student among his intake.• It's a charity thing - something to do with April Fools' Day.