I used to think that your birthday changed every year, and that your parents just decided what day it was.
A childhood belief about birthdays

others believed things like this
- My parents have the same birthday -- month, day, and year. I though all mom's and dad's had the same birthday.— Cait
- My birthday was on a different day every year, assigned by my parentsAnon
- iwas told that if u cry on your birthday you would not get any bigger 4 a whole year— steve
- i used to think that if i didn't celebrate my birthday each year my younger sister would catch up with me— q
- i thought I got one year older when my family sang the birthday song.— I am the ghost
- My sister uesd to think that you did a whole year's worth of growing in the night before your birthday.— Richard Davies