The phrases " on tomorrow," " on today," and " on yesterday " are commonly heard in the southern region of the United States. They are acceptable in casual speech and other informal contexts, but should not be used in formal contexts such as academic writing. american english - Origins and history of "on tomorrow", "on today ...

Understanding the Context

In old books, people often use the spelling "to-day" instead of "today". When did the change happen? Also, when people wrote "to-day", did they feel, when pronouncing the word, that it contained two Change from to-day to today - English Language & Usage Stack Exchange I think it is a good question. When there is yesterday morning and tomorrow morning, why have an exception for this morning (which means today's morning)?

Key Insights

Yes, idiom, but I actually do like idiomatic extensions like these - as long as everybody knows what is meant and no grammar or semantic rules are violated... Neither are clauses, but "today in the afternoon" is grammatical (adverbial phrase of time), while "today afternoon" is not. I would also suggest "this afternoon" as a more succinct and idiomatic alternative to "today in the afternoon". word choice - 'Today afternoon' vs 'Today in the afternoon'? - English ...

Final Thoughts

The last example means something different, though. “What day is (it) today?” refers to the day of the week, not the date.