Origin and usage of "day of"

You say, The implied meaning is "adv. on the aforementioned day (of some event)".

I disagree. It means, "on the day": a subtle but meaningful difference. "On the day" is the more common abbreviation.

"Should we pick a restaurant now?" "Nah, we can decide on the day."

"On the day, the weather turned out to be terrible."

Although I'm Brtish, I understood the meaning of the phrase immediately. Probably because (like @Greybeard) I'm used to hearing "End of!"

Indeed, this expression is idiomatic to a number of native speakers. Significantly, it’s part of my dialect.

The expression is an ellipsis of “the day of [the event]”, and there are more possible variations:

• The night of
• The evening of
• The morning of
• The afternoon of
• The week of

(But not “end of”, which I believe is an expression only used in British English. In any case, it’s not idiomatic in my dialect.)

It’s the same as expressions like “the night before” and “the day after”: what used to be a preposition is now an adverb.

The internet is rife with examples. It doesn’t look to me like the expression isn’t really specific to any one region of the US.

What does it mean when someone has a preliminary trial but the day of they don't bring them to court — Avvo, Maryland

but I didn't know about it until the night of — Columbus College of Art and Design, Ohio

the police were not called the night of — Lawyers.com, Florida

If you're unable to join the evening of, we plan to post the audio recording on the South House Website. — Dartmouth, New Hampshire

I'm going to call them the morning of; they're all busy and I want to make sure they see it. — Hollywood Reporter, New Jersey

Encourage your student the night before and the morning of — Alachua Schools (download), Florida

On the afternoon of, she canceled, pleading fatigue and an impending sore throat. — NYT, Missouri/New York

Joe didn't know until the week of. — The Morning Call, Illinois

...And then there’s the fact that many other examples I found online were all about the same thing: wedding days. (These examples were mostly “the day of”, though there’s a significant number for “the morning of” as well.) Unlike previous examples, you’ll see the expression used extremely frequently, even multiple times on the same page. It’s used so much that it’s spawned an adjective (e.g. in “day-of coordinator”). In fact there are even wedding planners with this name like The Day-Of, Colorado and The Day Of Company, New York. Some examples:

• Veilevents, Kansas
• Something You On, Pennsylvania
• Knot Too Shabby, North Carolina