Coincidentally, every diet seems to, "start on monday". From the Atkins diet to the Sugar Buster. The Monday Diet is home to many many different diets that are have been thrown on the "back burner" by overweight, lazy individuals. In addition, this diet typically lasts 1-3 days.
"Hey Phil, you know man I really don't think you should be eating a second Big Mac, those things are packed with the cals buddy."
"It's ok man, my diet starts monday, I gotta enjoy myself over the weekend"
"Ahh the Monday Diet, gotcha"