Sentence meaning?

"Now a secret must be imparted. Professor Pnin was on the wrong train. He was
unaware of it, and so was the conductor, already threading his way through the train to
Pnin's coach. As a matter of fact, Pnin at the moment felt very well satisfied with himself.
When inviting him to deliver a Friday-evening lecture at Cremona—some two hundred
versts west of Waindell,
Pnin's academic perch since 1945—the vice-president of the Cremona Women's Club, a Miss
Judith Clyde, had advised our friend that the most convenient train left Waindell at 1:52
P . M ., reaching Cremona at 4:17; but Pnin—who, like so many Russians, was inordinately
fond of everything in the line of timetables, maps, catalogues, collected them, helped
himself freely to them with the bracing pleasure of getting something for nothing, and
took especial pride in puzzling out schedules for himself—had discovered, after some study,
an inconspicuous reference mark against a still more convenient train (Lv. Waindell 2:19
P . M ., Ar. Cremona 4:32 P . M .); the mark indicated that Fridays, and Fridays only, the two-
nineteen stopped at Cremona on its way to a distant and much larger city, graced likewise
with a mellow Italian name. Unfortunately for Pnin, his timetable was five years old and
in part obsolete."

What does the bold sentence mean in this context? And commas make this sentence very hard to read; Fridays indicat what exactly?

Thanks!