English Language Day (13 October)

What? Never heard of it.
Who...? Well, it was started by The English Project, of Winchester (UK).
"whose long-term aim is to establish a visitor attraction devoted to the English language in the city."
(I assume that last phrase means that the proposed visitor attraction will be "in the city [Winchester]"; rather than some sort of sociolinguistically-oriented attraction devoted to "the English language in the city" [urban English] or perhaps [Winchester English].
http://www.englishproject.org/
Why this day? Well (and I quote from Crystal):
...on this day in 1382 Sir Henry Green, the chief justice of the King's Bench, formally opened a new session of Parliament using English, instead of the traditional French. He stated the reasons for the assembly:
That the King [Richard II] was desirous to know the Grievances
of his Subjects; and particularly, that he might, by their
Advice, redress any Wrongs that had been done to Holy Church;
also, to reform all Enormities, especially about the Manner of
accepting Petitions in Parliament.
The first part sounds as if it might have been just a heresy-hunt; but the second was a real change. The Statute of Pleading (1362) put an end to 300 years during which petitions/pleas in any court had to be written in French, a language never known by most English people.
https://en.wikipedia.org/wiki/Pleading_in_English_Act_1362
Henry IV was the first English king since the Norman Conquest whose mother tongue was English. He made a speech to his people in English at his coronation, on 13 October [!] 1399 -- feast-day of the Anglo-Saxon saint/king Edward the Confessor.