The first of April, some do say,
Is set apart for All Fools' Day.
But why the people call it so,
Nor I, nor they themselves do know.
But on this day are people sent
On purpose for pure merriment.
--Poor Robin's Almanac, 1790
APRIL FOOL'S DAY CRIME FICTION
The Marsh Madness by Victoria Abbott
The Case of the April Fools by Christopher Bush
April's Fool by Edna May Ciesclwicz
April Fool by William Deverell
The April Fool by Robert J. Fields
April Fools’ Day Murder by Lee Harris
April Fool Dead by Carolyn Hart
A Remarkable Case of Burglary by H. R. F. Keating
The Confidence Man by Herman Melville
April Fool’s Day A Novel by Josip Novakovich (not quite a mystery but with mystery elements)
The Cruelest Month by Louise Penny
April Fool's Day by Jeff Rovin
The New Adventures of Sherlock Holmes, Vol. 3: The April Fool’s Day Adventure and The Strange Adventure of the Uneasy Easy Chair by Anthony Boucher and Denis Green.
YA: The April Fool's Day Mystery by Marion Markham