Indeed, it is dögum because of á (but I think I told you so in my
first reply ;) )

"eina nótt" is in the accusative because it is a temporal adverbial
phrase, and for some reason such phrases are in the accusative.

Other people may give you further explanations, as I unfortunately do
not have as much time as I would like to study this wonderful language
!

2009/7/5 nikolai_sandbeck <nikolai_sandbeck@...>:

>
>
> Hej, i got two examples here of time, the first one is dativ used (dögum):
>
> Þat var á dögum Haralds konungs ins hárfagra, Hálfdanar sonar ins svarta, at
> sá maðr kom skipi sínu til Íslands í Breiðdal, er Hallfreðr hét.
>
> and the second one is accusative used (nótt):
>
> Ok eina nótt dreymdi hann, at maðr kom at honum ok mælti: "þar liggr þú,
> Hallfreðr, ok heldr óvarliga. Fœr þú á brott bú þitt ok vestr yfir
> Lagarfljót. Þar er heill þín öll,"
>
> Which one scould i use? :s
>
> Is the dögum in dative only becuase of á?
>
> Well I dont study at university, so a detailed plain word explaination would
> be nice. but of course I'll be happy for alle the help i can get :b
>
> thanks
>
>