Collecting old calendars was a hobby of mine. One day my school organized a school picnic and told us that our class would be going on July 30, 2007.
When I went home, I accidentally saw the 2005 calendar and noted that 30th was a Saturday. But the 30th of July according to the 2007 calendar was a Monday.
When I reached there on Saturday morning there was no one in the school. I saw a friend of mine passing by and asked him why no one had turned up.
He drew my attention to my mistake. Since then I have stopped collecting old calendars.