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?

  • 2
    I can't think of an example where ある日 and [一日]{いちにち} can be used interchangeably.
    – chocolate
    Jan 14, 2018 at 5:20
  • Also including furigana with {} behind the Japanese characters and including links or sources to where you found your translations is helpful for people wanting to answer you question.
    – Yannick
    Jan 14, 2018 at 6:09
  • Sorry, I assumed people very knowledgeable in Japanese didn`t need furigana, or the links. I guess depending on how frequently used the terms are, its possible for a native speaker to need them, knowing how crazy kanji is.
    – Great Gas
    Jan 14, 2018 at 6:44
  • As certain kanji combinations can have different readings and usages depending on those readings, it is important to specify which reading, hence furigana necessity.
    – BJCUAI
    Jan 14, 2018 at 6:55
  • An example of this being in my answer. 一日{いちにち} is not interchangeable, while 一日{ひとひ} is (in literary Japanese). It is always good practice to include furigana if not at least for people less proficient in Japanese. Remember, this site is here to provide questions and answers for the whole community not just one persno.
    – Yannick
    Jan 14, 2018 at 8:38

1 Answer 1


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. 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


    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


    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


    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.


    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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