After extensive research about personality types, birth charts, and characteristics of people born on the mentioned days, as well as based on the post by the Julie and the Phantoms official account on instagram (here), these are some of the JATP characters' birthdays (and yes, the show's timeline was also taken into account):
Willie: February 20th (Pisces)
Flynn: March 22nd (Aries)
Alex: July 7th (Cancer) [x]
Luke: August 13th (Leo) [x]
Julie: September 20th (Virgo) [x]
Reggie: December 21st (Sagittarius) [x]
Carrie: December 30th (Capricorn)
The research in question was conducted by yours truly, @sweetdayme4427, @screamsingayfear, & others, but mostly by Aleks β‘
We will not be taking any criticism at this time, thank you, but @sagrandr will be happy to answer any questions!
