Shichi-go-san is a ceremony to celebrate children's growth. On November 15th, people celebrate their children's growth at the age of 3 and 5 for boys, and 3 and 7 for girls. The name Shichigosan comes from these ages. "Shichi" means seven, "go" means 5 and "san" means 3.
Although it is not true for today, babies were supposed to get shaved their head till they were at the age of three. After the age of three, people thought they were not infant any more. The celebration was called "Kamioki", meaning putting hair on their head.
In Edo era, as for boys celebration, they wore "Hakama" to show that they were not child any more. "Hakama" was a formal ware for man. And for girl’s celebration, they wore "Obi" instead of a string to tie around the waist. At these ages, people treated them as a man or woman.
Nowadays, these differentiations with age have gone away, but still we celebrate children's growth on "Shichi-go-san".
Background History: Shichigosan is a Japanese custom that began in farming villages, where children often died young. The custom took root in the samurai class during the Edo period (1603-1868).