Q: My supervisor sends out emails with sentences like “Debby will not be at work on tomorrow” or “We will meet on this afternoon.” Is it appropriate to say or write “on tomorrow” or “on this afternoon”? What is the rule for this?
A: The only “rule” here is common usage. If enough of us begin using “on” with “tomorrow” and such words, it will become an accepted idiom.
For now, though, it’s still unusual enough to grate on most people’s ears. In the best English, we don’t hear expressions like “on tomorrow” or “on this afternoon.” Here’s why.
The words “today,” “tonight,” and “tomorrow” already include the implied preposition “to.” In fact, they were once written as “to day,” “to night,” and “to morrow.”
Later, hyphens were added (as in Macbeth’s “sound and fury” soliloquy), then the hyphens fell away and the words were joined. To use the additional preposition “on” with these is redundant.
By extension, it’s redundant in sentences like the other one you cite to use the preposition “on” as well as the demonstrative adjective “this” with “evening,” “afternoon,” “morning,” etc.
The Oxford English Dictionary explains that the expressions “this morning, this afternoon, this evening now always mean ‘the morning (etc.) of to-day.’ “
(I’m leaving aside emphatic usages like “The night was memorable because it was on this evening that he proposed” or “On this particular morning we discussed the company’s future.”)
Within its entry for the preposition “on,” the OED has this definition: “Indicating the day or part of the day when an event takes place.” (This is the sense in which your supervisor uses it.)
The OED‘s citations, dating from Old English to the present, generally pair the preposition with holidays (“on Wintanceastre,” “on Cristesmessa,” “on All Soules Day”), days of the week (“on fridæi,” “on saterdei,” “on Sundays,” “on Tuestay,” and so on), or dates (“on the 29th”).
The OED does note, however, that in US and in Irish English, the preposition “on” is sometimes “used with tomorrow, yesterday, etc.” – but it adds that this usage is apparently redundant.
I hope this sheds some light.