Almost always the day I created them or during a time I think suits their birth. I have a soft spoken, earth friendly, hippy girl who I wanted to be born in spring and picked a date from there. Kurei was the date I updated her back in 2007. Sometimes I do Zodiac signs (they run strongly in my family) if I'm feeling a reason for it or I can't settle on a date. But the date I create them is the most fun for me cause that is their literal 'birthdate'.