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Based on what I was able to find, both means "one day" except ある日 is used when
referring to a certain day. Could I use these words interchangeably? or are
there situations where using one is unacceptable/improper?
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2018-01-14T05:13:23.873
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meaning word-choice
What is the difference between ある日 and 一日
The meaning of one day you found is the one in the sense of "a certain day",
i.e. referring to a day without saying exactly which. An example in English
is:

```

    One day, he went out and never came back.
    
```

So please keep this in mind. This is not the 'one day' from the sequence of
one day, two days, three days etc.

So now let us go through the examples as outlined in [this dictionary
entry](https://dictionary.goo.ne.jp/thsrs/12867/meaning/m0u/). The meaning of
all the synonyms is 'a certain day' or 'one day' as explained above (I will
use both these in that meaning interchangeably).

  * ある日{ひ} is probably the most prominent example here. It is used like

> 数日{すうじつ}たった **ある日** 一通{いっつう}の手紙{てがみ}が届{とど}いた
>
> **one day** , after numerous days passed, a letter arrived

  * 一日{いちにち} cannot be used interchangeably with ある日 but by preceding it with a noun and の it can be similar in meaning

> 春{はる}の **一日** 、ひなびた山峡{さんきょう}を訪{たず}ねた
>
> **One day** in spring, I visited the rural ravine

  * 某日{ぼうじつ} should be clear from the characters employed to quite literally mean 'some day', in that sense I guess you could interchange it with あるひ

> 某月{ぼうげつ} **某日** 、誰{だれ}それに会{あ}ったというメモが書{か}き残{のこ}されていた
>
> The memo said that some month **and some day** , a certain so-and-so was met

  * 一日{ひとひ} is actually very rarely used and always needs furigana since one would normally read this as いちにち. But this could be exchanged with ある日.

> **ひと日** 、郊外{こうがい}に遊{あそ}ぶ
>
> **One day** , we played in the outskirts.

Now it is important to notice the last paragraph of that dictionary entry: ある日
is by far the most widely used and pretty much the sole one used in
conversational Japanese. The latter three do appear in novels every now and
then, but this is really confined to the literary world (especially the latter
two).

So the answer to your question: Can one use them interchangeably

  * ある日 and 一日{いちにち}: **no**. To make it work you have to construct something like 春の一日 and then it is only literary Japanese.

  * ある日 and 一日{ひとひ}: **only in literary Japanese**.
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answer