This painting celebrates the Japanese holiday Boy’s Day.
Tango no Sekku, Boy’s Day or Boy’s Festival is held each year on the fifth day of the fifth month (May 5). Parents of boys display replicas of ancient armor (or samurai helmets), and koi-nobori, fabric windsocks shaped like carp, are flown on long poles on the roofs of houses to show how many children live there.
(Nussbaum, Louis Frédéric et al. "Tango no Sekku." Japan Encyclopedia. Boston: Harvard University Press, 2005. 948. Print.)