This is something I've been wondering for some time now. And this is also not related to relative temporal nouns, I understand why you cannot use に with those.

Consider these:

○「2月14日にデートするつもりです。」"I will go on a date on February 14".

○「2月14日『は』忙しいですよ。」"I'm busy on February 14".

○「2月14日に外食します。」"I'm eating out on February 14".

○「2月14日『に』予定がない。」"I have no plans for February 14". Notice how ない could be considered an i-adjective.

○「2月14日に会いましょう。」"Let's meet on February 14".

○「4月23日にもう一度来てね。」"Come again on April 23".

○「11月14日に生まれました。」"I was born on November 14".

But if you wanted to say:

☓「2月14日『に』忙しいです。」that's wrong.

1) Is there a grammatical reason why this is wrong or unnatural?

2) Is this related somehow to i-adjectives? (What about adjectival nouns?)

3) Where else can I expect this behavior?

4) When should I use に to mark specific times? It doesn't seem to work exactly as it does in English for example.

Please, if you have any kind of resource, I will appreciate it. Even those in Japanese are useful to me, thanks.

Here are some threads I read that might be somewhat related:

what is relative time in Japanese language?

Why is に marking time and not で?

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忙しい or other states of being (not limited to i-adjectives) have some duration for which the state remains. On the other hand, に stands for a pinpoint time during the date within the day.

I mean, a day in particular has 24 hours, that sounds like a range to me

14日 can be a time period that has width, but 14日に indicates a point in the 14th day. Moreover, the essential sense of に is making something adhere to a point so that it doesn't move randomly, which doesn't really get along with a state of being something, which ranges beyond the point.

If any, 14日に動いている means that you can confirm some movement in the moment of observation. (For some reason, 忙しい as it is is still unnatural, compared with 忙しくなっている, even in this usage.) That's different from what you intended, isn't it?

Thanks for additional info.

P.S. Whether it's ungrammatical or just unnatural is obscure, but as long as you mean that the state of being busy ranges for a day, it's close to misuse of grammar. However, you could use it as long as you only refer to the moment the state is observed, though a non-past form of i-adjective is unsettling, compared with other forms.

2月14日に忙しい sounds like habitual present ("He is busy on every Feb.14") because it's in non-past besides an observed fact.

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    添削させていただきましたが、“during the date”がよくわからなかったです。 – Darius Jahandarie 2 days ago
  • I'm really struggling with what is being said here. It seems like this is implying that you cannot use に because "states of being" have a range of duration. I fail to see how a state having a range means it cannot be pinpointed to a day, I mean, a day in particular has 24 hours, that sounds like a range to me. Is this a grammatical constrain?. And what about the example with 予定がない、is that allowed because is a negative form? hence being a non-state? – TyrantRC 2 days ago
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    The に in 〜に〜がある/ない would normally not be treated as the time marker but rather the dative subject marker or location marker, which sort of could carve an exception for it. Of course that’s not a terribly satisfying explanation since it feels a bit arbitrary, but. – Darius Jahandarie 2 days ago
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    Other examples of stative predicates that don’t work with [time]に are things like 3時に走っている or 3時に仕事中だ. – Darius Jahandarie 2 days ago
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    English is quite flexible in allowing you to say “I will be [state] at [time]” but Japanese seems to be more restrictive... or rather just requires you to say it a different way. – Darius Jahandarie 2 days ago

Let me show my idea to explain this really difficult question.

After the “(Absolute time)に” we basically expect something happening or something changed. Adjectives and adjectival nouns are just for describing status of something and that’s far from happening, that's probably why it doesn’t fit in the sentences like your example.

In order to grasp this idea further let’s see the exceptions of the rule.

(good) 「いつ車が必要ですか?」「月曜にほしいです」

ほしい(want) is classified as adjectives in Japanese. But as English does, it has verb-like feature. So this sounds natural unlike most of adjectives.

(bad) 「14日に忙しいです」

(good) 「14日に忙しくなります」

As @user4092 said, the latter one is totally good though the stem of the verb phrase is adjective. "忙しくなる" means "getting busy". It indicates the status changing on that day.

These are the adjective examples which make sense. I can show opposite examples using verbs which don’t make sense.

(good) 「11時にお客さんが増えます」

(bad) 「11時にお客さんが増え続けます」

The former example meaning "Customers will increase at 11p.m." sounds very good. But "増え続ける" (keep increasing) sounds bad. This is because keep doing situation actually don't change anything.

(occasionally good) 「来月に川が流れます」

(good) 「来月に川が流れ始めます」

Similarly, the above former example meaning "The river flows on next month" sounds good as long as it means now the river doesn't flow. Since "(Absolute time)に" attract some changes or happenings, it gives us a feeling like "it doesn't flow currently". So for such a case, 来月に川が流れ始めます is better.

(good) 「23日に来て」

(bad) 「23日に来ないで」

Interestingly, 23日に来て sounds good but 23日に来ないで is not good. That's because nothing happens in the negative sentence basically!! We use 23日来ないで to tell "Don't come on 23rd."

(However, 「23日に来ないで」 is not always bad because we sense different meaning from this sentence...)

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