Why every year is cursed with Friday the 13th

Some 20 million Americans fear this day and date, making it the most feared day in history. Unfortunately this has to be a yearly occurrence, and we prove this with some simple number theory. On the plus side, the most it can happen in a year is 3 times.

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[MAGMA]

Fun fact-The rates of car crash and other kinds of incidents actually decrease on Friday 13th due to people's fear of going outside

[mikefochtman]

As a child, I played around with my dad's "forever calendar" one day. You just set the day of the week for Jan 1st and it would show you the days of the week for every month of the year (with a separate display for leap years). It was then that I noticed certain months always had the same day-of-week patterns (Sep days always align with Dec, Apr always align with Jul for example). Just a fun little bit of 'numerology' to impress your nerdy friends with. lol

[TheChamp]

Friday the 13th is still better than Monday the whatever.

[kevinslater]

I did this same calculation as a teenager but the math I used was looking at a bunch of calendars and realizing I was screwed.

[warlock]

Even easier than this, and no math - all you have to do is observe from the general calendar that there's somewhere in the year a Monday the 13th, a Tuesday the 13th, a Wednesday the 13th, etc. Therefore, no mater what day of the week is the first of the year, somewhere there will be a Friday the 13th..

[rjones]

If the first day of the month, falls on Sunday, it will have a Friday 13th.

[_Loki__Odinson_]

Love this buddy
Please never stop
You're fun to watch every single time

[michaelcoloradogambler]

This was the basis for a competition question in a high school math contest I participated in 25 years ago! And I'm pleased to say that I got the question right. 😊

[newz]

One aspect not in your proof and probably obvious to some but not everyone… the day of the week that the calendar year starts on doesn't matter because it simply shifts all numbers by the same amount. Since every day of the week (the modulo result) is represented, the order in which the days are represented will be shifted but the frequency of the numbers (at least one, at most three) will not change.

[barttemolder]

Super simple proof / funny coincidence:
Each month in the 7 month range May to November starts at an unique weekday with every weekday appearing once, so logically the 13th of each month will be on an unique workday as well.
And this is the same for leap years as February isn't in this range.
As each year has the months May to November in it there must be a Friday the 13th in it just like any other weekday the 13th.

[snowfloofcathug]

Fun fact, with there being 97 leap days per 400 years they repeat perfectly in that span. 2201 / 05 / 04 will be the same day of the week as 1801 / 05 / 04. Doesn’t matter what date you pick, if you check 400 years in the future, the weekday will be the same

Also, there’s something called the Doomsday Algorithm that says that 4/4, 6/6, 8/8, 10/10, 12/12, 9/5, 5/9, 11/7, and 7/11 will always fall on the same day within the same year

[ohboi]

I am not from any western country but learned about this from the western cartoons

[Lex_Verse]

Would've been totally awesome if you released the video on Friday the 13th.

[mrcryptozoic]

I dispute only the "lost business" assertion. The business is just postponed (or preponed) to another day. Same business, same transaction, same money.

[JoeRoe-wpmu]

More specifically, every year has a Friday the 13th within the range of May through November. This is because these seven months (May, June, July, August, September, October, November), all begin on different days of the week, meaning that one of them must have a Friday the 13th each year.

[trevorbradley]

A couple more edge cases to check!

September 1752 only had 19 days in the Julian calendar. But 1752 had 2 Friday the 13ths, so you're still good.

The Julian calendar also changed the first month from March to January. Using the old Gregorian calendar would change January and February to 0 and 1 modulo 7 respectively... And again you're still good!

[programaths]

Once you have the first remainder, you can add one for the month after February and take the modulo 7 of the result.
You can also use the calendar itself as a calculation device.
So, you know that the first 13 is 6 modulo 7 because 13=14-1
Then, you scan the same column and stop on the 10th of February. You need to add 4 days to get the 13.
So 6+1+2, the modulus of the 13th of February is 2.

That way, you swapped division by 7 with scan, see what to add, rinse, and repeat!
The nice thing is that the biggest number you can deal with is 7.

[wesleydeng]

4:25 For leap years, You don't have to recalculate. Only need to add 1 to the non-leap year's Mar. - Dec. numbers and change to 0 if the result is 7.

[DJSchreffler]

Fun fact: If you look at the 400-year Gregorian Calendar cycle, it ends up being a whole number of weeks.
So if you count up all the 4800 months, you find that the 13th falls on Friday more often than any other day of the week.

[ohboi]

QUICK QUESTION: A goat is tide to a rope of a circular fence of radius 1m .The goat grazes grass half the area of the circle . Find the length of the rope.