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EDIT: Still looking for a complete answer. Please read the edit below.

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?](https://japanese.stackexchange.com/questions/68550/what-is-
relative-time-in-japanese-language)

[Why is に marking time and not
で？](https://japanese.stackexchange.com/questions/68856/why-is-%E3%81%AB-
marking-time-and-not-%E3%81%A7)

* * *

**EDIT:**

After thinking about this for a few days, I find that my initial assertion on
relating a native English speaker way of thinking with a native Japanese
speaker was fundamentally wrong.

This might be more similar to English that I initially thought, for example we
cannot say "I will be busy at/in February 14" which would might very well be a
better analogy to using に in the Japanese sentence.

However, I'm still looking for an answer to **why does using に in a sentence
like「2月14日に予定がない。」is allowed?** (consider that ない can be both a negative of a
verb and an i-adjective). I assume that this is related to an implication in
language from reading Mitsutoshi's answer, the implication being that your
state from being "busy" to being "non-busy" has changed (an implied instance
of changing states), however, this is faulty because the same could be said
for the sentence like「2月14日に忙しい。」where the difference between using に vs は
would be an expressed "something has changed and I'm busy that day".

I would like to hear more about this from other people.
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grammar particle-に i-adjectives time
Why can we not use に to mark absolute time with i-adjectives?
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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answer
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」
>
> (not good) 「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 **は** 来ないで to
tell "Don't come on 23rd."

However, 「23日に来ないで」 is not always bad.

In order to grasp this negative case clearly, let me explain further.

Suppose an answer during conversation between 2 people.

A: "You said you go to English school everyday, right? Can I go with you
tomorrow?"

> (good) B: 「14日『は』レッスンがない」
>
> (bad) B: 「14日『に』レッスンがない」

That's because the question under this context focuses on a event to go to the
school together. These 2 people are to pay attention to the event. However, B
wants to say "the school is day off tomorrow" so the event will never happen
on 14th. Thus 「は」 is good but 「に」 is bad.

Similarly,

A: "You look very busy recently. Do you get enough sleep? Are you OK?"

> (good) B: 「そんなに忙しくないよ。14日『は』予定がない」
>
> (bad) B: 「そんなに忙しくないよ。14日『に』予定がない」

2 people focus on the amount of plans of B. A worry about B who looks very
busy but B wants to say "I'm not so busy". In this case, a plan which is the
concerning event of them is absent on 14th. Thus 「は」 is good but 「に」 is bad.

However, there are many cases 14日『に』予定がない sounds more natural.

A: "You've been busy so we haven't go on a date recently. When are you free?"

> (good) B: 「14日『は』予定がない」
>
> (better) B: 「14日『に』予定がない」

In this example, I think 「に」 sounds better because it sounds like B is looking
forward to the date!! That's because the concerning event of them is the date
and "no plan" is nearly equal to "to go on a date" in this case.

A: "I know you're very busy. Please tell me when you are free."

[![plans for a very busy
man](https://i.stack.imgur.com/eaGDH.png)](https://i.stack.imgur.com/eaGDH.png)

> (good) B: 「14日『は』予定がない」
>
> (better) B: 「14日『に』予定がない」

Similarly, 「に」 sounds better because for a very busy man who has plans like
the image above it's kind of happening that there is no plans all day long. So
"no plan" is nearly equal to something happening in this case too.
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