I used to believe, bizarrely, that everyone had a different birthday. Eg. because I had 'bagsyed' Sept 11th, no-one else had it.
This was very stupid of me because I knew how many days there were in a year, and that there were millions more people than that in the world.
Still, I believed it.
A childhood belief about birthdays

others believed things like this
- I waited for the mail to come on my 11th birthday so that I could get my letter to Hogwarts... It never came.— Charlie
- My parents have the same birthday -- month, day, and year. I though all mom's and dad's had the same birthday.— Cait
- When I was 6 I believed that on nice summer days you can see the world spin if you just look hard enough :)Anon
- My birthday was on a different day every year, assigned by my parentsAnon
- I used to think that your birthday changed every year, and that your parents just decided what day it was.— Daniel
- iwas told that if u cry on your birthday you would not get any bigger 4 a whole year— steve