This tidbit of history is interesting because how exactly would anyone know the day of a birth definitively? What was stopping parents from lying about the child's actual birthday in order to spare them the stigma of a "bad" day and give them a good day instead?
I also read that one birthday could pave your way into a Calmecac school even as a lower class. What was stopping parents from basically lying their way into a University?
What do we know about Aztec Divination by /u/400-Rabbits touches on the "technicalities" associated with birthdays