Well I think some superstitions were 'invented' to protect people from doing foolish, stupid or downright dangerous things.
I mean walking under ladders does have lots of danger potential.......
Staring at the sun during an eclipse will do real physical harm to your eyes. Possibly even sending you blind, so whoever decided to say that it was unlucky was certainly not only telling the truth, but potentially saving the eyesight of many many uneducated/stupid and crazy people.
Friday the 13th is a little harder to fathom...........
I mean Fridays....Well Jesus died on Good Friday...(What made that Friday so good I wonder? I bet it wasn't so good for Jesus!)
Eve is said to have tempted Adam with the apple on a Friday.
The flood is said to have begun on a Friday too.
Perhaps it might be more pertinent to ask what religion has against Fridays???

Although it seems most people agree through the years that Fridays are THE unluckiest days.............................No-one seems to EVER say why!!!!
In 1656 it was written "Now Friday comes, what old wives say, is of all, the week's unluckiest day". So by this time Friday was obviously quite well established as being unlucky.
"There are many respectable merchants who will not do business or take medical advice on Friday as it is well known to be the unluckiest of days." This was written in 1831.
Friday is considered a very unlucky day to start new things. Projects, activities, ventures or endeavours.
Needleworkers around 1883 would routinely set aside work they had almost finished on Thursday and put a few stitches in a new project so they would not have to start a new piece on a Friday.
A group of farm workers in 1933 were written in the newspaper as having worked overnight on Thursday UNPAID, to take the first cut line from every field, so they would not have to begin a new cut on a Friday!!!
Fishermen in the 1880's would not lay the keel of or launch a ship/boat on Friday. And no new voyages were ever considered on a Friday.
Marriages begun on Fridays were said to be doomed to failure.
And of house moves made on Fridays it was said "A Friday move will never stay, doomed to move again next day!"...........................
Thirteen is said to be unlucky and once again we go back to the bible for the 13 people present at the last supper, of which Judas Iscariot was the 13th guest to be seated.
So early in the 18th century it was said that if 13 people were to sit down to dinner together one of them was destined to die within the following year. A special chair with a stuffed creature or doll seated on it was to be found in many restaurants, then if a group of 13 were to sit to dine it could be brought out and placed at the table to be the 14th guest!
And many people will still not set sail with a crew and passenger compliment numbering 13.
By the late 19th century the superstitions about the number 13 had become widespread, with people going out of their way to avoid anything associated with the number 13, whether car number plates, door numbers, room numbers, hotel or high rise floors, desks, or children.
Many floors and rooms were misnumbered so the occupants could pretend they were NOT in room or on floor 13.
And the 13th of the month became as unpopular as Friday for doing, beginning or ending 'things'.
And of course if those two uhappy times were to co-incide then tragedy and calamity were surely to befall the unwary or foolhardy!
If anyone is interested the claim that the superstition surrounding Friday the 13th began with the arrest of the final Grand Master of the Knights Templar, Jaques Demolay, on Friday October 13th, 1307, is a 20th century invention with absolutely no valid facts to back it up!!!
The earliest known expressions of Friday the 13th as a day of evil luck are to be found in both the Los Angeles Times and the New York Times in 1908. As a short piece about an ill fated Senator from Oklahoma who foolishly tried to tempt fate by introducing 13 bills to the senate in Washington on Friday the 13th of March of that year. Both newspapers agreed that as they had all been put forward on this day there was really little hope for any of them!

One last thing I found was a piece in a New York paper dated 1913 telling the story of a minister who offered to marry free of charge any couple willing to risk getting married on FRIDAY THE 13TH.
There are no updates later to say whether or not he had any takers!!!

I hope this helps answer some of the questions about Friday the 13th, but unfortunately I think mostly it is going to be a case of................
No-one really knows how it started and no-one really knows why. It just is and always has been.......... Even though it is documented as a fairly new thing.

Darkness