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 is 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: