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.

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