OC. I computed the weekday for every 13th of every month from 1900-2099 using the Gregorian calendar and plotted the distribution.
Results (n = 2,400 months):
- Monday – 342 (14.25%)
- Tuesday – 343 (14.29%)
- Wednesday – 342 (14.25%)
- Thursday – 343 (14.29%)
- Friday – 344 (14.33%)
- Saturday – 343 (14.29%)
- Sunday – 343 (14.29%)
Why this happens (short version): calendar arithmetic + leap-year rules skew the weekday distribution of the 13th ever so slightly toward Friday.
Data & code: GitHub Gist
by Interesting-Sock3940
4 Comments
But is it statistically significant? I need to know that p < 0.05.
The calendar runs on a 400-year cycle, so doing this for 1900 – 2299 would be more appropriate. And the results are 688 Friday; 687 Sunday and Wednesday; 685 Monday and Tuesday; 684 Saturday and Thursday.
But 2100 – 2299 actually has more Wednesday the 13ths than any other day!
Source: I ran OP’s code with the years changed.
Ugh just my luck – I was born between those two dates.
I just wish we would transition to the international fixed calendar.