Kodomo no Hi (??????????????? meaning "Children's Day") is a Japanese national holiday which takes place annually on May 5, the fifth day of the fifth month, and is part of the Golden Week. It is a day set aside to respect children's personalities and to celebrate their happiness.
via en.wikipedia.org
Traditional celebration of Kodomo no Hi includes the annual marking of the posts with the height of each child, which around here is probably easier than trying to hoist a family of carp up the flagpole. The Festival marks the edge of summer, the original name literally meaning "the edge of five" which I'll wager is a term instantly understandable to any four year old.