OK Day (23 March)

That should be "'OK' Day", of course. An "OK Day" would be a so-so, not-bad day. This one is about the word.
23 March 1839, Boston Morning Post -- first known appearance of the word (in the form "o.k."), standing for "all correct".
Allen Walker Read, etymologist and lexicographer, clarified its origins in a fashion for whimsical eye-dialect abbreviations among American wits of the early 19th century, and put to rest several other theories of its origin that had been proposed.
https://en.wikipedia.org/wiki/Allen_Walker_Read
https://en.wikipedia.org/wiki/OK
Who thought it needed a "Day"all to itself?
Allan Metcalf, who wrote a book about it (2011).