adverbs - Does "It snowed hard Monday" require an "on"?

I came across an English learner writing

It snowed hard Monday.

After saying that it didn't snow on Friday and Saturday.

It didn't quite feel right to me.

I'd be okay with

It snowed hard.

or

It snowed Monday.

Is it okay for there to be two things ("hard" and "Monday") modifying the snowing without an "on" breaking things up a bit?

Answer

In North American Engish, Monday can be used as adverb to mean on Monday, in the same way Mondays is used to mean on Mondays, on each Monday.

I have looked for sentences similar to the ones shown in the question on the Corpus of the Contemporary American English, and I found the following ones. (I looked for sentences containing "[vvd] hard [npd1]"; that is the COCA's way to look for sentences containing "[past tense] hard [weekday]" where the part between brackets is a token that allows to precisate the category of the word.)

We're going to check in now with a couple of towns that have really borne the brunt of the Flood of' 93. The Quincy area, Quincy, Illinois, and West Quincy, Missouri, hit hard Friday night when an important levee broke there.

We skied hard Saturday to clear our bodies of Friday night's brewtasting toxins.

"In my mind, they make up for some of the balls I hit hard Friday night that were right at people," Ripken said.

The Corpus of the Contemporary American English has a total of three sentences that match the search terms "[past tense] hard [weekday]" (1 in the "spoken" section). It doesn't have any sentence matching "[past tense] hard on [weekday]" or "[past tense] hard [preposition] [weekday]."

As comparison, there are 215 sentences matching "[past tense] [adjective] [weekday]" (43 in the "spoken" section), and no match for "[past tense] hard [preposition] [weekday]." In the first case, there are sentences containing "came late Sunday," "called late Thursday," or "came clean Monday."
There are also 16 sentences matching "[past tense] on [adjective] [weekday]" (5 in the "spoken" section), with sentences containing "came on Super Tuesday," "shopped on Black Friday," or "attended on Good Friday."