Paraskevidekatriaphobia: the Fear of Friday the 13th

Above: Still from the film Friday the 13th showing Jason Voorhees before he got pumped up and had a makeover.

Non haec sollemnia nobis,
Has ex more dapes,
Hanc tanti numinis aram
Uana superstitio…imposuit,
-King Evander
Virgil, Aeneid, VIII, 185-188

Neither these rituals, these formal feasts, nor this altar at this holy place of power have been imposed on us by empty superstition.-my translation

Or, more succinctly:

These rites, these altars, and this feast, O king,
From no vain fears or superstition spring,- John Dryden's translation

Virgil places superstition in a place outside of established religion. Virgil is stating that any belief which is empty (uana, vain, empty) is not valid and is unworthy to be compared to the devout practices devoted to the honor of the gods. Such thoughtlessly performed activities must be considered superstition by definition. Of course, most people today would place the religious practices of Classical Times in the category of superstition, devoutly performed or not. We might say then that in our day any irrational practice intended to placate invisible beings, avert bad luck or facilitate good luck and is outside one's accepted religious belief could be described as superstition. But Virgil seemed to be specifically consigning superstitio to those almost unconscious irrational behaviors performed without much thought.
At the beginnings of culture various practices of this kind may have been interchangeable with incipient religious practices. At some point the priest class has to sort out which practices will make the cut for formal acceptance in the religion. Some practices, known as "survivals" may have begun as superstition and trended into the realm of cultural custom, such as hand-shaking or toasting. Days of rest may have developed from tabu days when it was considered sacriligeous or risky to do anything. Friday the 13th is kind of a tabu day for me. I usually put in for a vacation day at work to avoid going out. Do I really believe something will happen if I go out. No. I don't think so; I just like 3 day weekends. But that's how these things can get started.
And do I keep an elderberry bush by the back door because it is traditionally anathema to witches or because I enjoy the shrub? Well, I aint got nothing against those witches.