It’s the last day of Golden Week, a series of three holidays here in Japan. This year is was April 29th (Shouwa Day), May 3rd (Constitution Memorial Day),. May 4th (Greenery Day) and today, May 5th (Children’s Day).
Because girl’s have Hina Matsuri in March, Children’s Day in May is really for the boys.
These carp streamers are flown wherever there are boys, like at this child care center that’s near my school. The black one is for the father, the red one represents the mother, and each additional carp is another child. Blue -> Green -> Purple / Orange (depending on region). I’ve not seen either of them here, possibly because I also have seen the carp for any families that have more than two boys.
And the correct name of these carp are Koinobori (Koi – carp, noburi – streamer).