Coming-of-Age Day

Coming of Age Day (成人の日, Seijin no Hi) is a public holiday in Japan held annually on the second Monday of January under the Happy Monday System. It is held in order to congratulate and encourage all those who have already reached the age of maturity between April 2 of the previous year and April 1 of the current year, and to help them realise that they have become adults. Festivities include coming of age ceremonies (成人式, seijin-shiki) held at local and prefectural offices, as well as after-parties among family and friends

Source: https://en.wikipedia.org/wiki/Coming_of_Age_Day