Fait mathématique: 1561 is the number of series-reduced trees with 19 vertices.
Culture générale: 1561 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: April 9th is the day in 1992 that a U.S. Federal Court finds former Panamanian dictator Manuel Noriega guilty of drug and racketeering charges.