Coming of Age Day for Yui and Moa
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.
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. – Taken from the Wikipedia page. You can get more information here.