Koinobori (The large carp at the top represents the father, the second carp represents the eldest son, and additional carp are added to represent each subsequent son with color and position denoting their relative age.)
Kodomo no Hi (こどもの日, 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. It was designated a National holiday by the Japanese government in 1948.
The day was originally called Tango no Sekku (端午の節句 ), 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 5th on the Gregorian calendar. The festival is still celebrated in China, Taiwan, Hong Kong and Macau as the Duanwu Festival or Duen Ng Festival (Cantonese), in Korea as the Dano Festival, and Vietnam as the Tt Đoan Ng on the traditional lunar calendar date.
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."(It also means 5) 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.
Until recently, Tango no Sekku was known as Boys' Day (also known as Feast of Banners) while Girls' Day (Hinamatsuri) was celebrated on March 3. In 1948, the government decreed this day to be a national holiday to celebrate the happiness of all children and to express gratitude toward mothers. It was renamed Kodomo no Hi. There is some concern that, despite its renaming, it is still Boys' Day and it is inappropriate that Boys' Day is a national holiday, while Girls' Day is not.
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 and the kabuto are symbols of a strong and healthy boy.