I have one of those “every day’s a holiday” calendar, and it says that tomorrow is Customer is Wrong Day. I even googled it after I saw that, and it’s real and has been around since at least 2010 (and maybe earlier; I didn’t look at all the results).
Unfortunately, I don’t think that my workplace celebrates it.
Every employee working that day should be gifted a coupon that allows them to tell off a customer of their choice that day with full managerial support.