Use of に with 先週


I saw this sentence on a Japanese teaching site:

> 事故は先週に起こった  
>  The incident occurred last week

I feel very uncomfortable with the に on 先週. [This
post](https://japanese.stackexchange.com/questions/84536/difference-
between-%E6%AF%8E%E6%97%A5%E3%81%AB-and-%E6%AF%8E%E6%97%A5) and [this
post](https://japanese.stackexchange.com/questions/68550/what-is-relative-
time-in-japanese-language) seem to back up my discomfort. Is this sentence
ungrammatical/unnatural or am I missing something?




As a native speaker, I also feel uncomfortable with 事故は先週に起こった, which can be
said to be a little unnatural but probably cannot be said to be ungrammatical.
先週 plays the role of an adverb in this sentence, so its natural sentence will
be:

  * 事故は先週起こった。

But as a writer, I would like to avoid this, because I don't like the awkward
sequence of Chinese characters of 先週 + 起. So I like this sentence transcribed
like this with the use of punctuation marks.

  * 事故は(、)先週、起こった。
  * 先週、事故は起こった。(very natural to me)

This point might be trivial for other Japanese, but I am always wondering how
to use this kind of noun phrase used as an adverb. In order to avoid this
(discomfort), I prefer the following to the above sentence.

  * 事故は先週のあいだに起こった。(a little longer, but very natural to me)

After answering this question, I hit upon the following sentence.

  * 先週 **に** 起こった事故は、すごかった。

This sentence with に in it is, no doubt, grammatical and quite natural. I have
no idea of the cause of the difference in naturalness and grammaticality
between this and the above sentence (事故は先週に起こった).




As a native speaker, I also feel uncomfortable with 事故は先週に起こった, which can be
said to be a little unnatural but probably cannot be said to be ungrammatical.
先週 plays the role of an adverb in this sentence, so its natural sentence will
be:

  * 事故は先週起こった。

But as a writer, I would like to avoid this, because I don't like the awkward
sequence of Chinese characters of 先週 + 起. So I like this sentence transcribed
like this with the use of punctuation marks.

  * 事故は(、)先週、起こった。
  * 先週、事故は起こった。(very natural to me)

This point might be trivial for other Japanese, but I am always wondering how
to use this kind of noun phrase used as an adverb. In order to avoid this
(discomfort), I prefer the following to the above sentence.

  * 事故は先週のあいだに起こった。(a little longer, but very natural to me)

After answering this question, I hit upon the following sentence.

  * 先週 **に** 起こった事故は、すごかった。

This sentence with に in it is, no doubt, grammatical and quite natural. I have
no idea of the cause of the difference in naturalness and grammaticality
between this and the above sentence (事故は先週に起こった).

