Last week, this poster appeared outside the public bath in my neighborhood, reminding us all that Iris Fighting Day was nearly upon us. Since 1282, when intrepid samurai repelled the Mongol Hordes (no, seriously, there really were Mongol Hordes!), May 5 has been the day when fathers take time away from battling spreadsheets and conquering new markets to take a bath with their sons.

But it’s not just any old bath – iris leaves must be employed to lend a fighting spirit to the experience. The Japanese words for “contest” and “iris” sounds alike, and because iris leaves are conveniently sword-shaped, it’s a no-brainer to put the two together for manly bathtime entertainment.