The day was originally called Tango no Sekku (端午の節句?), and was celebrated on the 5th day of the 5th moon in the lunar calendar or Chinese calendar. After Japan's switch to the Gregorian calendar, the date was moved to May 5.[1] The festival is still celebrated in China, Taiwan, Hong Kong and Macau as the Duanwu Festival or Tuen Ng Festival (Cantonese), in Korea as the Dano Festival, and Vietnam as the Tết Đoan Ngọ on the traditional lunar calendar date. It was originally for boys but has since been changed to include both boys and girls.
Sekku means a season's festival (there are five sekku per year). Tango no Sekku marks the beginning of summer or the rainy season. Tan means "edge" or "first" and go means "noon".