I wanted to share this in the Once Guild, but for some reason I can't post new threads there, so here we go.
Recently Twice shared messages to Once on the anniversary of when the fandom got the name (04.11):
It's so cute that they not only celebrate their anniversary but also the anniversary of the fandom! I've never seen a group do this 
Translations (found here on reddit):
Nayeon: Happy birthday, ONCE. Here, it's seaweed soup (*Koreans eat seaweed soup on their birthday) Let's drink it and get one year older.
Jeongyeon: ONCE, ONCE! Happy birthday. I'm so happy that we have similar birthdays. ONCE must be happy. You have two birthdays. That's why you like TWICE.
Momo: Happy birthday, ONCE. Thank you for always being with us and being supportive. It's so precious. Let's have fun together in the future. It's your birthday, so eat something delicious.
Sana: Happy happy Once Day! I hope ONCE will have a more fulfilling and happy day than anyone else. And I hope it's a day where you think, "I'm glad ONCE is here. No, I'm glad I liked TWICE."
Jihyo: Happy birthday, ONCE. Today is the only day of the year! I hope ONCE is the happiest. ONCE, thank you always. Let's stay healthy and happy together for a long time. Love you ONCE
Mina: ONCE, you've grown up a lot. Let's continue to grow healthy together. Happy birthday!!
Dahyun: ONCE~ Happy birthday. Eat a lot of delicious food since it's your birthday. ONCE is already 9 years old. Let's make a lot of good memories together
Chaeyoung: Happy birthday, ONCE. It's already our 9th birthday. Thank you for supporting us and giving us precious words and love. Let's continue to congratulate each other.
Tzuyu: Dear my ONCE. You got a year older. Let's make more memories together.
