Or perhaps you've been in a store and seen evidence of them too...
But what are they for, you might be asking yourself!
They're called Koinobori (the translation in English is literally just "carp streamers"!) and they are hung to celebrate Children's Day (こどもの日 or Kodomo no Hi), which is one of the holidays in Golden Week. Originally, Children's Day was actually Boy's Day, and male children (and fathers) were celebrated - but that was overhauled and now the holiday celebrates all children and their parents. It's celebrated annually on the 5th of May.
What is the meaning behind the colors of the carp flags?
Tradition dictates that the black carp symbolizes the father of the family, and the red carp (sometimes they'll be pink, too) symbolizes the mother. Any other carp that are hung are representative of the children in the family - and the color and positioning of those are ranked according to age.
What does a fish have to do with Children's Day?
Good question - I had the same thought when I first saw these flying everywhere at this time of year and was told they were to celebrate children! Basically, when Children's Day was originally Boys Day, the carp was chosen as a symbol since they are seen as a strong and spirited fish - it's in their nature to swim upstream and fight the currents. When you think about strong wills and spirits, children definitely come to mind -- so I can understand now why they chose the carp to identify with!
Where can I get the carp streamers if I'm wanting them as souvenirs?
They can be found all over the place - I've seen them at 100 yen stores, at Don Quixote stores, at toy stores...even at some supermarkets! Basically, just keep your eyes peeled and you're bound to stumble across them at this time of year!
As a parent I love the entire concept of celebrating children, and the whole point of Children's Day is to celebrate their happiness and their personalities. We'll be flying the carp flags at our house for sure!