Shichi-Go-San (七五三) or “Seven-Five-Three” ceremony is a traditional rite of passage for 3 & 7 year-old girls and 3 & 5 year-old boys. This event is held annually every November 15th when they give thanks to the health of their children and pray for continued happy and healthy futures, the practices and traditions have developed and been adapted over the years in modern practice. The number 15, which is the aggregate of 7, 5, and 3 is also considered an especially auspicious number.
Generally in modern day celebrations of shichi-go-san, the children are dressed traditional outfits, with girls often appearing in kimono and boys in hakama, and they visit their local Shinto shrine with their parents, where they pay a visit to the tutelary deity.
The roots of the tradition of praying for the health and well-being of children have a few varieties, but below is the one most believe.
The notion is that it came about during the Heian period, when court nobles used to celebrate the passing of their children from infants to middle childhood. These numbers were especially celebrated as they are now, due to the luck perceived in odd numbers.