Carp fish (koi) are used as festive decorations during Children's Day ( Kodomo no hi). This is a Japanese national holiday which takes place annually on May 5, the fifth day of the fifth month. It is a day set aside to respect children's personalities and to celebrate their happiness.
The day was originally called Tango no Sekku and was celebrated on the 5th day of the 5th moon in the lunar calendar or Chinese calendar. After Japan's switch to the Gregorian calendar, the date was moved to May 5.
Sekku means a season's festival (there are five sekku per year). Tango no Sekku marks the beginning of summer or the rainy season. Tan means "edge" or "first" and go means "noon". In Chinese culture, the fifth month of the Chinese calendar was said to be a month for purification, and many rites that were said to drive away evil spirits were performed.
Before this day, families raise the carp-shaped koinobori flags (carp because of the Chinese legend that a carp that swims upstream becomes a dragon, and the way the flags blow in the wind looks like they are swimming), one for each boy (or child), display a Kintarō doll usually riding on a large carp, and the traditional Japanese military helmet, kabuto. Kintarō boy and the kabuto are symbols of a strong and healthy boy.