i was planning to make a thread about 1999 so i'm happy to see this, long story short it is a Christmas song and was their first hit song so ever since the band always does something related to the song each year around these dates
for example in 2021 they released the English version of the song
Hitsujibungaku announces 1st English single "1999"
goes out on November 24, it's an updated English version of their 2018's Christmas song "1999" very unexpected news, is rare for this group to even use English words in their music let alone a full song you can listen to the 2018 version here btw Moeka Shiotsuka is pretty fucking cool...
hallyuplus.net
original song
also they did the ED song of the current season of Jujutsu Kaisen
and the band's new album "12 hugs (like butterflies)" went on sale earlier this month