In Japan, to celebrate Boys' Day (鲤のぼり/こいのぼり, or "Day of the Offering of Sons" in Japanese) on May 5, every family with a boy hangs a carp flag. This custom was started in the Edo period as a custom of the Dragon Boat Festival (端午の节句/たんごのせつく) on the lunar calendar to pray for the early success of the boy in the family, and it is related to the Chinese story of the carp jumping over the Dragon Gate, but now it has been changed to May 5 on the Gregorian calendar. Carp flags are hollow carp made of cloth or silk and are divided into three colors: black, red, and blue, with black representing fathers, red representing mothers, and blue representing boys, and the number of blue flags representing the number of boys. The Japanese believe that the carp is a symbol of strength and courage and expresses the desire of parents to have their children and grandchildren become brave and strong warriors. The day is designated as a national holiday based on the principle of "respecting the child's personality, seeking the child's happiness, and thanking the mother". (I'm just a Baidu encyclopedia porter)