I call the explanation “WeakDaze.”
Because of the reaction people have when they wake up and realise it's a workday again and the weekend is over, the first day of the week is called Moanday.
Many people too busy to cook on the second day of the week just open a can of beans. Hence the day is known as Tootsday.
By the third day of the week, people are wondering when they can ever find the time to get everything done this week that they need to, hence the day is known as Whensday.
Too bleary to even count properly, people think it's only Day Three of the week on the next day, therefore it's erroneously called Thirdsday.
On the last day of the workweek, people often go out "for a few" after work. By the time they get home, they're too tired to cook anything elaborate, so they just throw a piece of meat, chicken, or fish in the skillet. That's why the day is known as Fryday.
Saturday night all the singles let loose. There's a lot of sexual hijinks. It's pretty obvious why the day is called Satyrday.
And on the last day of the week - and the weekend - people look at all the items on their to-do lists that didn't get crossed off, groan aloud, and make themselves promises they won't keep. Therefore the day is called Soonday.