I was at the library doing some other research last night, so I took the time to also look into this subject via the standard
koine Greek lexicon,
A Greek-English Lexicon of the New Testament and other Early Christian literature by Bauer, Danker, Arndt and Gingrich.
On
paraskeue:
in our lit. only of a definite day, as the day of preparation for a festival; acc. to Israel's usage . . . it was Friday, on which day everything had to be prepared for the Sabbath, when no work was permited Mt. 27:62. . . . For Christians as well [paraskeue] served to designate the sixth day of the week . . . and so in Mod. Gk. For Christians it is a fast day, as the day of Jesus' death [Didache 8:1]. (emphases in original)
As BDAG points out, if you type the word "Friday" into
Babelfish and translate it from English to Greek, what do you get?
Paraskeue.
As a quick discursus, here's the passage from the Didache that BDAG references:
And let not your fastings be with the hypocrites, for they fast on the second and the fifth day of the week; but do ye keep your fast on the fourth and on the preparation [i.e. paraskeuen] (the sixth) day. (8:1-2)
Should be self-explanatory: the writers of the Didache, an early Christian handbook describing the practices that obtained in the late 1st century, instructs Christians not to imitate the "hypocrites" (i.e. the Jewish unbelievers) by fasting on Tuesday and Thursday; rather, to be separate from them in part by fasting on two different days: Wednesday and Friday. The use of
paraskeue to denote a day of the week, rather than a special day of preparation for a special feast day, is undebatable.
Again back to BDAG, on
prosabbaton:
the day before the Sabbath, i.e. Friday, used to explain the word [paraskeue] Mk 15:42. (emphases in original)
In conclusion: the linguistic evidence is that Mark was using the normal words for Friday when talking about the day Jesus was crucified.