What was different the morning after Willie's Friday meeting with his friends?

For the first time, Willie hadn't wet the bed.

When Willie woke the next day, there was something altogether unusual about the morning. He lay in bed for some time and stared up at the ceiling trying to puzzle it out. Finally he gave up and clambered out of bed. It was only when he started automatically to strip it that he realized what it was that was so different. There was no need for the sheets to be washed that day. They were dry.