This is ...

This is the festival day for children who are aged thirteen years old and
cerebrate their coming of age on 13th April in the present calendar or 13th March in the lunar calendar in Japan.

In another word, this is the period that every senior elementary student goes to a junior high school after spring vacation in March
so that it is so-called chie-mairi in Japanese or going to a shrine for wisdom in English.
Boys wear haori-hakama and girls wear furisode as well as on the coming of age.
They might wear iro-tomesode, homongi or komon because wearing it s a longer period use even after wedding.
It will be the first time for most of girls to put make up ,sometimes heavy make up.
Girls in Kyoto put make up and wear a Maiko costume for visiting a shrine.