It’s his mortal birthday, not explicitly stated in canon. It’s what Anne Rice has told us, so not everyone accepts it as canon. As for his vampire birthday, we don’t know the exact date, but I headcanon he wouldn’t celebrate it. I don’t think any of the vampires celebrate or even mention the exact dates for when they were turned, except Claudia (L/L seem to celebrate the night she was turned).

AR has said she based Lestat on her husband, Stan Rice, whose birthday was Nov. 7.