Because the word for Japanese iris was pronounced same as the word for martial spirit (both were pronounced "shobu") and also the shape of leaves of Japanese iris remind people of swords, tango was determined as the sekku for boys and people prayed for the healthy growth of boys.
鎌倉時代ごろから「菖蒲」が「尚武」と同じ読みであること、また菖蒲の葉が剣を形を連想させることなどから、端午は男の子の節句とされ、男の子の成長を祝い健康を祈るようになった。