Had to open a portal into hell to retrieve these but here you go in case you're curious.
January 1 - Letty
January 5 - Tojiko
January 8 - Kyouko
January 10 - Mamizou
January 11 - Marisa
January 17 - Eirin
January 20 - Aya
January 23 - Tenshi
January 30 - Tewi
February 2 - Mystia
February 3 - Komachi
February 7 - Miko
February 24 - Merlin
February 26 - Sakuya
March 1 - Yuyuko
March 3 - Luna
March 4 - Patchouli
March 8 - Suwako
March 12 - Reimu
March 22 - Mokou
April 4 - Cirno
April 5 - Yuugi
April 7 - Ichirin
April 10 - Futo
April 19 - Hatate
April 28 - Ran
May 3 - Rumia
May 5 - Flandre
May 17 - Iku
May 24 - Akyuu
May 31 - Kosuzu
June 1 - Remilia
June 6 - Nitori
June 9 - Alice
June 24 - Orin
July 3 - Koishi
July 6 - Youmu
July 10 - Lunasa
July 26 - Reisen
August 6 - Wriggle
August 7 - Kanako
August 9 - Sunny
August 21 - Utsuho
August 23 - Murasa
August 25 - Meiling
August 27 - Chen
August 29 - Minoriko
September 4 - Hina
September 5 - Byakuren
September 7 - Keine
September 14 - Kaguya
September 24 - Eiki
October 2 - Kasen
October 5 - Rinnosuke
October 19 - Sanae
October 27 - Yukari
November 2 - Kogasa
November 7 - Satori
November 13 - Parsee
November 15 - Shizuha
November 18 - Nazrin
November 22 - Yuuka
November 28 - Suika
November 30 - Nue
December 6 - Star
December 8 - Seiga
December 14 - Shou
December 18 - Lyrica
December 29 - Yamame
It's been almost a decade though so you're not getting any explanations other than "it fits their personalities".