I was at the post office last Saturday to have a package guaranteed to arrive in Denver on Monday. When I filled out the form, I checked "next-day delivery," thinking that because the mail doesn't run on Sunday, Monday would actually be a one day delivery. The lady told me that they would deliver the package on Sunday, but it would be very expensive. I told her that it didn't need to be there until Monday, anyways, and I changed it to 2 day delivery.
Boring story, but I had no idea that the post office was actually capable of delivering packages on Sunday.