May 5th is Children's Day (こどもの日 or kodomo no hi) in Japan! This day celebrates the health and growth of children as well as expressing gratitude to mothers and celebrating the family as a whole.
In the past, Children's Day was known as 端午の節句 (tango no sekku), which a seasonal festival at the beginning of the fifth month. The character for "go" sounds like the word for "five" and thus the celebration occurs on the fifth day of the fifth month. The celebration originally was for the sons of the family (Girl's Day or Hinamatsuri ひな祭り is on March 3rd), praying for them to grow healthy and strong. Thus, the decorations and celebrations for this festival center around samurai even though today it celebrates all children.
Carp streamers or koinobori (鯉のぼり) are flown to pray for the health of children in the family. In the past, they were flown for the sons of the family, but now they represent the parents and all children in the family. The red koi (carp) traditionally represented the oldest son, while the blue and other colors represent the younger sons. Today, the black carp represents the father, the red or pink carp represents the mother, and blue, green, and orange carp represent the children.
Families often display samurai armor, either the full set or just the headpiece, along with a bow and quiver of arrows and a katana. These symbolize strength and vitality. This display is usually only set up in families with boys (families with girls set up doll displays in March).
Kintaro (金太郎) is a popular folk tale about a young boy who was raised in the mountains and befriended the animals who lived there. He was an active, super strong little boy who carried a hatchet and wore only a bib with the character for gold (kin 金) on it. His adventures include fighting monsters and demons, sumo wrestling with bears (and winning!), and helping woodcutters to fell trees. Kintaro is based on a real person who lived during the Heian, who as an adult became a personal retainer to a samurai due to his impressive strength. It is easy to see why this folk hero is a prominent figure in Children's Day, and in addition to decorations featuring Kintaro, often young boys are dressed in his signature "kin" bib for photos.
Children's Day food centers around symbols of strength. Oak trees (kashiwa) are strong and solid and thus they fortify the children who eat their leaves. Mochi cakes (glutinous rice cakes) filled with red bean paste cooked and wrapped with oak leaves are called 柏餅 kashiwa mochi. Glutinous rice wrapped in an iris or bamboo leaf in a long cylindrical shape are called ちまき chimaki. Another Children's Day non-food tradition is for boys to soak in a bath of iris leaves and roots (called 菖蒲湯 shoubuyu) which prevents illness and misfortune. A competition is also called shoubu (written as 勝負) and thus this bathing was used in the Kamakura era to pray for samurai to win battles.
There is a song called 鯉のぼりの歌 Koinobori no Uta (The Carp Streamer Song) which is short and simple.
やね より たかい こいのぼり
おおきい まごい は おとうさん
ちいさい ひごい は こどもたち
おもしろそうに およいでる
Yane yori takai koinobori
Chiisai higoi wa kodomo tachi
Carp streamers are higher than the roof
The biggest carp is the father
The small carp are children
Enjoying swimming in the sky
Here is an animated video with the lyrics in Japanese, and here is a video montage of Children's Day decorations, plus a bonus song called せいくらべ Sei Kurabe (Comparing Heights).