Does the 13th most commonly fall on a Friday?
Let's examine all dates in the common era i.e. 1st January 0001 to 31st December 2020.
Start by generating a dataframe of the date, day of week, and day of month for every date in this period.
Let's summarise this in a new dataframe of the frequency of the various weekday/monthday combinations:
Which day of the week does the 13th most commonly fall on?
What are the most common weekday/monthday combinations?
So whilst the 13th of each month does fall most frequently on a Friday over this period, it's only by a tiny amount (6 times more frequently than any other day).
However, there is nothing special about the frequency of this weekday/monthday combination, as 28 other combinations of weekday/monthday occur with the same frequency (3476) over the period.