A day late, but oh well:
Yesterday was not only Cinco de Mayo, it was also Children's Day in Japan.
For well over a thousand years it used to be called Boys Day, but shortly after the end of WWII, the Japanese government decided to have a single holiday celebrating children, so they chose May 5th.
The most visible representation of Boys Day is koinobori. Traditionally when flown, the top large black koinobori was supposed to represent the father; then under that was a large red koinobori representing the mother; and finally the smaller koinobori representing the sons. I guess nowadays the smaller koinobori represent both the sons and daughters.