A fictional holiday mascot like the Easter Bunny or Santa Claus. Cheese Weasel Day is April 3rd, or the following Monday if the 3rd falls on a weekend. On this day, supposedly the cheese weasel leaves a slice of cheese (most typically American cheese) under the keyboard of those who've been good during the past year to find when they arrive at work. Sometimes there are also greeting cards bearing related poetry. This holiday has been observed since at least 1992, and seems primarily associated with computer geek culture.
This morning when I got in to work, I was asked, "Did the Cheese Weasel visit you last night? Have you been good?"
There was a slice of Kraft American cheese under my keyboard.
"I guess you've been good then."