Iku 1/9
Suu 1/23
Ahko 2/5
Naddy 2/11
Mimimi 3/3
Nano 3/14
Momiji 4/8
Kusuri 4/18
Rentarou 5/1
Mei 5/10
Hahari 5/12
Uto 5/16
Hakari 6/22
Eira 8/3
Yaku 8/9
Rin 8/10
Yamame 8/11
Kurumi 9/3
Mai 9/6
Karane 9/9
Chiyo 10/1
Momoha 10/8
Meme 10/10
Kishika 10/10
Shizuka 11/1
Iku, Yaku, Mimimi, and Kurumi all have birthdays that stem from their names (see Suu’s intro chapter).
Momoha has 10/8 because 108 earthly desires in Buddhism. Nano has 3/14 because of pi. Suu’s is 1/23 because numbers.
Naddy’s is 2/11 because that’s National Foundation Day, when Japan’s first Emperor supposedly ascended to the throne in February 11, 660 BCE.
The birthday problem states that assuming you pick random people, once you assemble a group of 23 people there is a 50% chance that two of them share the same birthday. There are 24 now, and indeed Meme and Kishika were born on the same day.
Edit: More.
As mentioned below, Rin (8/10 -> Ba/io) and Karane (9/9 -> "~
nain dakara ne") also have birthday puns. They were changed in the translation for readability.
Mei’s birthday is May 10, which is apparently Maid Day in Japan. Mai is 9/6, Younger Sister’s Day. Kusuri is 4/18, Invention Day. Shizuka is 11/1, National Author’s Day.
Hahari’s birthday is May 12, which was Mother’s Day in 2019 (the year in which 100GFs started publication).