Anpan is a delicious snack consisting of a regular ol' bun (the pan) filled with red bean paste jam (the an). According to this long, illustrated history of Anpan Day, KIMURA Yasubei (who apparently looked like Santa Claus) and his son Eisaburou presented anpan, which they had recently invented, to the Meiji Emperor on this day in 1875 when he came down for flower-viewing at the Mito clan shimoyashiki, which I think eventually got swallowed up into Sumida Park.