I've heard the opposite. You lay off on a Monday so you have 5 business days to call recruiters. You give a promotion or bonus on a Friday so the employee can celebrate on the weekend and won't slack off the rest of the week.
Promote on Saturday so that they're not distracted on a workday.
Specifically: incompetent bosses mis-citing a study supposedly saying it's a good idea to fire at the end of the week.