Five Saturdays, Sundays and Mondays in December… perception, knowledge and weirdness

The other day one of my friends posted something on his Facebook wall that I thought was interesting.

This year December has 5 Mondays, 5 Saturdays and 5 Sundays. This happens once every 823 years. This is called money bags, so share it and money will arrive within 4 days. Based on Chinese Feng Shui.

Okay, I dismissed the "money bags" nonsense straight away, and something about that "this happens once every 823 years" seemed off, but one can't deny the weirdness of this month having five Saturdays, Sundays and Mondays, right? How often does that really happen?

Well, it happens a lot. Any month that has 31 days will always have five occurrences of three different days in it, because a week is seven days long, and 31 / 7 = 4 weeks + 3 extra days. So that means that, every year we have seven instances where that happens. Which three days they are is solely dependent on which day the month starts on, so to refute the "once every 823 years" bit, all we have to do is find the next December that starts on a Saturday, which is 2018. Six years, not 832.

So, Facebook post debunked, friend corrected and I'm sure I'll only see it a billion more times this month.

But I'm still stuck on why it seemed weird in the first place, why something that happens pretty much every other month rang unusual enough that reasonably intelligent people could think that it only happened every 832 years. About all I can come up with is that, unless we have some specific event tied to a day of the week that makes having five of them unusual (like if you're paid every other week on the same day, and some months you get three checks, or if you work at a church and know that some months you get five Sunday offerings) there's something in our brains that just assumes that there are always four occurrences of every day in a month.

Until I sat down and worked out the fact that having five Saturdays, Sundays and Mondays this month is actually pretty common, it truly did seem unusual and, even now that I'm fully aware of the facts, it still seems a little weird. Not sure what to attribute it to, though.

Thoughts?

Originally posted by adjensen

But I'm still stuck on why it seemed weird in the first place,

Because someone said that it was. In this current society of 'little to no attention span', many people like to allow others to do their thinking for them.

Just thinking/saying, "Yeah. Wow. That is weird."
instead of actually taking a few seconds to think about it yourself.

I think I got that moneybags email 2 years ago.

It seemed weird because it was framed as being a rare thing and without the foresight of 0.3 seconds mental analysis it does seem unusual. Even everyday truths can be made to look amazing. For example, did you know that the entire adult population of Guatemala (when averaged out) have one only testicle and one ovary?

True story.