The word Koinobori means "carp streamer" in Japanese, and these are flown everywhere in the end of April, beginning of May to celebrate "Children's Day" on May 5th. It's the end of the "Golden Week" here, and that's a national holiday, where most locals have off work, schools are off, and many businesses close down.
A typical koinobori set consists of, from the top of the pole down, a pair of arrow-spoked wheels with a ball-shaped spinning vane, flying-dragon streamer that looks like a windsock.
The number and meaning of the carp socks or koinobori that fly beneath the streamer has changed over time. Traditionally, the set would contain a black koinobori representing the father, followed by a smaller, red koinobori representing his eldest son. If more boys were in the household, an additional blue, green and then, depending on the region, either purple or orange koinobori were added. After the government's decree that converted Boy's Day (Tango no Sekku) into the present Children's Day (Kodomo no Hi), the holiday came to celebrate the happiness of both boys and girls.
As a result, the red koinobori came to represent the mother of the family and it is not uncommon for the color to be varied as pink. Similarly, the other colors and sizes of carp came to represent all the family's children, both sons and daughters. At present, the koinoburi are commonly flown above the roofs of houses with children, with the biggest (black) koinobori for the father, next biggest (red or pink) for the mother, and an additional, smaller carp of a different color for each child in decreasing order by age.