What does shichi-go-san celebrate?

This is an event in which parents celebrate their children's growth.
On November 15, 3- and 5-year-old boys, and 3- and 7-year-old girls are taken to shrines where their parents pray for their future.
Boys often wear haori, or a half-coat. and hakama, or a divided skirt, and girls wear kimono.
Nowadays, some children wear suits or dresses.