This tongue torturer appears to come from the Greek words paraskevi, Friday, and dekatria, thirteen, with the suffix -phobia tacked on for the fear factor.

Word Spy

Hey,that could make a good movie title.

Oh no, that's today!

The Modern Greek word for Friday is interesting on its own, though: from Greek paraskeue 'preperation' (also a late meaning was the Friday before Passover Sabbath); from para- + skeue 'attire, equipment'. There's also a Saint Paraskeva in the Greek Orthodox Church.

And here I thought Freya's dag came after (W)odin's dag and Thor's dag.
Loki dind't get a dag.

And paa Dansk Shabbes is bathday: Lørdag.

why is it "dekatri" and not "triskaideka" as in the world "triskaidekaphobia" which is fear of the number 13?

..........................................................

triskaidekaphobia
noun
extreme superstition regarding the number thirteen.
ORIGIN early 20th cent.: from Greek treiskaideka ‘thirteen’ + -phobia .

why is it "dekatri" and not "triskaideka"
Probably because the knowledge of Greek is not as widespread as it used to be in the era of Alexander the Great

why is it "dekatri" and not "triskaideka"

Because Classical Greek has treiskaideka, dekatreis and dekatria for 'thirteen' (take your choice), and Modern Greek has dekatria.