"Day of the Book Smugglers" is Crystal's English, though Google
Translate gives me "Book Carrier Day".
It all goes back to a period (1863-1904) when the Russian Tsars
(starting with Alexander II) attempted to stamp out all forms of
Lithuanian language and culture in what was then their province of
Lithuania. A creepy governor, Count Mikhail Nikolayevich Muravyov (his
surname means "ants") was entrusted with carrying out this project.
https://en.wikipedia.org/wiki/Mikhail_Muravyov-Vilensky
Naturally it was a crime to publish, sell or read books in Lithuanian.
But various people managed to establish clandestine smuggling networks, obtaining Lithuanian books and periodicals from neighbouring countries
and distributing them. (One estimate is that more than 5 million items
found their way into the country during this time.)
March 16 is the birthday of Jurgis Bielinis, one of the leading
smugglers, who became something of a folk hero.
https://www.ramuva.lt/index.php/istorijos/musu-mintys/108-kovo-16-knygneio-diena
One site seems to indicate that "Su Knygnešio Diena!" is how you would
say "Happy Book Smugglers Day!" in Lithuanian. I mention this only
because it relates to a topic that came up recently on a.u.e. In place
of the ubiquitous holiday-greeting "Happy..." in English, Russian just
uses the preposition "S" (with) (plus the instrumental case of course).
I'm guessing that "Su" is the Lithuanian cognate of this.
Here's a page announcing last year's Knygnešio Diena:
https://www.facebook.com/photo.php?fbid=896288054958596&set=a.502140551040017&type=3&locale=lt_LT&paipv=0&eav=AfZ3QNvcsN8tZR5oZhKVoJbv88ZWXkSfYVZcCoGaT3UltCd5dlUR2VfHy2iE5KWZ4G4&_rdr
The celebrations don't look all that colourful or exciting, but it's a
nice idea, worthy of commemoration. Jim McCawley, who believed all
linguists should celebrate Hangul Day along with the Koreans, might have considered this one also worth a party -- people risking death to keep
their national language alive.
--- SoupGate-Win32 v1.05
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