Boy's Day is a Japanese national holiday which takes place annually on May 5 and is the final celebration in 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. It has been a day of celebration in Japan since ancient times. 

Source: Wikipedia

BOYS' DAY

Until 1948, Children's Day was known as Boys' Day (also known as Feast of Banners), celebrating boys and recognizing fathers, as the counterpart to Hinamatsuri, or "Girl's Day" on March 3. In 1948, the name was changed to Children's Day to include both male and female children, as well as recognizing mothers along with fathers and family qualities of unity. 

On this day, families raise the koinobori, which are carp-shaped windsocks (carp because of the Chinese legend about a carp that swims upstream and becomes a dragon that flies to Heaven, and the way the windsocks blow in the wind looks like they are swimming), with a black carp for the father, a red or pink for the mother, and one carp (usually blue, and sometimes additionally green and orange) for each child. 

Traditionally, when celebrated as Boys’ Day, the blue koinobori was for the eldest son with additional colors for younger siblings. Families may also display a samurai doll, sometimes riding on a large carp (often representing the Japanese folk heroes Kintarō or Momotarō), and/or the traditional Japanese military helmet, kabuto, due to their tradition as symbols of strength and vitality.

Kashiwa-mochi (sticky rice cakes filled with red bean jam and wrapped in oak leaves) and chimaki (sticky sweet rice wrapped in an iris or bamboo leaf) are traditionally served on this day.

Source: Wikipedia