Iris
Originally, Tango no Sekku was known as Ayame no hi, and was a day to purify the home by thatching the roof with irises, which had properties of repelling evil spirits (History).
In the Kamakura Period, Ayame no Hi became a day for boys, ostensibly for the reasons below:
Puns: The word shoubu has a few different meanings in Japanese:
- 尚武 - warlike spirit
- 勝負 - contest
- 菖蒲 - Japanese Iris
Visual puns: The iris looks somewhat like a blade.
Derived from this, there is a tradition to float iris leaves in a boy’s hot water bath in the hopes that he may become a great samurai. This is known as shoubu-yu.