As Yui and Moa turned 20 in June and July of 2019, they will have their Coming of Age Day ceremonies/celebrations on this day.
Suzuka had her’s on January 8th, 2018. Like many of these things with BABYMETAL, expect it to be kept private, and no pictures to appear online.
The pictures used are from the Tower Records posters that came with the release of Megitsune in 2013.
What is the Coming of Age Day?
For those unaware, the Coming of Age Day is a Japanese holiday held annually on the second Monday of January. It is held in order to congratulate and encourage all those who have reached or will reach the age of majority between April 2 of the previous year and April 1 of the current year, and to help them realize that they have become adults. Festivities include coming of age ceremonies held at local and prefectural offices, as well as after-parties among family and friends.
Nearly all the young women prefer to wear an elaborate kimono or furisode, a long-sleeved kimono for unmarried young women, on this day.
Others sources: Wikipedia page