As a child I would sing along with the Beatle's "Eight Days a Week" before I had a firm grasp of the actual number of days in the week. For years after, I thought I was miscounting since I couldn't think of the name of the other day of the week.
A childhood belief about general

others believed things like this
- I thought that "tomorrow" was one of the days of the week— Frank
- I always thought that after 1999 it would be 1990 again, but with different months and days of the week
— optional - I once refused to believe in Thursday. I'd heard there were seven days in a week, but could only name six.Anon
- I used to think that church songs were pop music. Well, we did sing them in church every week...— Roxy
- I thought the guy who got shot at the beginning of Gunsmoke was replaced every week by a live guy.— Monica
- Up until I was ten I believed that Yesterday was a day of the week— Kirsty