I used to believe that "Yesterday" and "Tomorrow" were actual days of the week, and that they both had a day of the week for which they were a double. I would ask my parents, "what day of the week is tomorrow?" and they would say "Thursday". And I would be outraged, because the last time I asked that question, they had said that Tomorrow was Monday, and now they were changing the story.
A childhood belief about lies

others believed things like this
- I thought that "tomorrow" was one of the days of the week— Frank
- I once refused to believe in Thursday. I'd heard there were seven days in a week, but could only name six.Anon
- Up until I was ten I believed that Yesterday was a day of the week— Kirsty
- I always thought that after 1999 it would be 1990 again, but with different months and days of the week
— optional - My neighbors used to believe you could never count to 100 in a day, it would atleast take a week.
— raceystacyUS - I used to want to be a garbage man when I was older because I belived they only worked one day a week— Oscar the Grouch