It’s 成人の日 today. It’s to celebrate whose who have reached the age of 20. Family and friends will gather to have parties. They may also attend ceremonies and giving a speech, organized by their schools or community. Boys and girls will dress up for the day. It is a standard for girls to wear Kimono和服, a traditional Japanese clothing. It explains why you will see so many people wearing Kimono as you walk in the street on this day.