Young people in Japan are considered to be adults when they reach 20, not 21. “Seijin no Hi” or “Coming of Age Day” is a national holiday in Japan held every year early in January. On that day we have a ceremony like a kind of collective birthday party to celebrate everyone who turned/will turn 20 sometime during the year (goes by Japanese school year system). Many female attendees wear “Furisode” for the ceremony.
