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In a story I read recently, there was a sentence:

Eines Tages fand Peter eine Schatzkarte!

And I wonder why it is "Eines Tages" and not "ein Tag", because "ein Tag" is what I would say normally. Is there any special reason, like a special poetic rule being used in stories or I'm wrong and the normal way is what is used there?

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Ein paar kleine Korrekturen habe ich vorgenommen, aber Kommata sind nicht meine Stärke - ich vermute jedoch, dass das Komma nach "Tages" falsch ist. Der Satz ist ansonsten aber richtig, während "Ein Tag" falsch wäre, ich kann leider nicht erklären wieso. Es ist auch keine Besonderheit bei Märchen. Eines Tages wirst auch Du diese Sätze souverän meistern. ;) –  user unknown Jul 23 '12 at 15:59
And I wonder why you reverted the changes. Well, it's up to you, ... –  Em1 Jul 23 '12 at 19:12
I have read many tales (Märchen) in german for my beginning to learn this language, and that was the question that came back constantly... Cuz the first words are always those: "Eines Tages...". I finally had just accepted that like an idiom. So many Thx for asking this. –  Stephane Rolland Jul 23 '12 at 19:45
@StephaneRolland In the end it is idiomatic saying and "just accepting" is what most German people do. Being a child you often hear the words "Eines Tages" but you likely will not ask why. One day (Eines Tages) you maybe will learn the grammar in school but I guess most German natives do not remember anything of all that stuff (though they nearly perfectly speak German ;)) –  Em1 Jul 23 '12 at 19:54
I figure "Eines Tages" translates like "one of these days", which makes the genetive aspect more intuitive. –  diN0bot Jul 25 '12 at 12:44
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4 Answers

up vote 15 down vote accepted

This is related to the question about "dieses Jahr, diesen Sommer", with the difference that you are asking for an indefinite time.

As Em1 already mentioned, it is an adverbial expression. You ask for such an expression using when (wann):

Wann fand Peter die Schatzkarte?

You want to answer:

Ein Tag.

But you answer with an indefinite time expression without a preposition. Then you have to use genitive (scroll down to "4) Expressions of indefinite time"). Genitive of "ein Tag" is:

Eines Tages.

Maybe the link in Em1 answer for the Adverbialgenitiv now gets clearer.

You also can use the preposition an when answering the question, like knut pointed out. Then you would need dative:

An einem Tag (fand Peter die Schatzkiste).

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Schön kompakt zusammengefasst. :) –  Em1 Jul 24 '12 at 7:43
Great, I was directed here to one question I posted. According to the answer to another question I had already posted (german.stackexchange.com/questions/7364/…), you'd need accusative to express time when no preposition is involved. So now I don't understand why sometimes you employ genitive and some other times accusative. –  martina Sep 10 '13 at 13:53
@martina: It's indefinite time without preposition (genitive). Definite time without preposition is accusative. –  John Smithers Sep 10 '13 at 19:52
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Eines Tages is an adverbial qualification (Adverbiale Bestimmung). There do exist several of adverbial qualification, in that case it is a temporal adverb. Strictly speaking in that example it is an indefinite time determination and that is one of the cases where you use an Adverbialgenitiv, i.e. the adverbial qualification is given in genitive. That's why we typically say:

eines [schönen] Tag[e]s

as it is also shown in the DUDEN.

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Thank you but that doesn't answer my question. –  Gigili Jul 23 '12 at 19:04
@Gigili Well, then what is your question? –  Em1 Jul 23 '12 at 19:13
Ein Tag wouldn't be correct at all. Ein Tag can be accusative or nominative, which cannot answer when-questions... you will need either a preposition or use the old fashioned genitive. That is not limited to Tag but also holds for Jahr, Monat and all the names. Most of the time it will be accompanied by a preposition but for days the genitive works. Anyway... a thing (like day) alone in accusative or nominative or dative cannot answer the question when? ever in German. –  Emanuel Jul 23 '12 at 21:56
@Gigili "Ein Tag" == "A day" as knut points out in his answer. You can say "Was für ein schöner Tag" == "What a beautiful day". –  Em1 Jul 24 '12 at 9:53
@ Gigili: no you can't... I mean not in German. It should be an einem Tag. No exceptions there. Saying "ein Tag" as answer to "when?" is wrong. "Ein Tag" can only answer "What?" or AT TIMES "For how long" –  Emanuel Jul 24 '12 at 11:28
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If you translate eines Tages it is some day or someday. Ein Tag is a day.

With ein Tag you express one day (not two or more), or a specific day. An einem Tag fand Peter eine Schatzkarte would say, there is a specific day.

With Eines Tages fand Peter eine Schatzkarte! you express there was a day, but it does not matter, which specific day it was. It was anytime.

Eines Tages is more vague than Ein Tag.

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Note, that "one day" is also a possible translation for "Eines Tages" and please remove the comma after "Eines Tages"... You should also mention that "An einem Tag" is dative. –  Em1 Jul 23 '12 at 19:48
@Em1 I removed the comma (it was a cut+paste from question). In another comment you wrote One day you maybe will learn the grammar in school but I guess most German natives do not remember anything of all that stuff - Yes, you are right. Whats the Dativ? ;) My answer is no answer about grammar, it's just a comment, how a native speaker understands the sentence. –  knut Jul 23 '12 at 20:06
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"Eines Tages" means the same as "an einem Tag" (to answer the question When?),

and not the same as "ein Tag" (which answers the question What?).

What the English phrase "One day" expresses is "At one day..." (something happened).

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How "ein Tag" answers the "what" question? "when? Ein Tag"! –  Gigili Jul 24 '12 at 9:37
An example: Question: "Where is he?" Answer: "In his room", not just "His room" - which would be the answer to "Which room is this?". It is the same with time: You don't say "Wednesday I do (something)", but "On Wednesday I do (something)". –  The_Fritz Jul 24 '12 at 10:35
@The_Fritz But you can say "Wednesdays I do ..." and that's actually the genitive structure. –  Em1 Jul 24 '12 at 11:42
"Ein Tag" and no other noun neither is able of answering the question Wann? all by itself in the German language. You need either case or preposition to make it work. –  Emanuel Jul 24 '12 at 19:49
@Emanuel: In German, maybe. But your example was in English where it doesn't work. –  Gigili Jul 26 '12 at 18:48
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