This is the 253rd most frequent Icelandic word.
vikunni
"Vikunni" means "the week" in English, in the dative case.
The word 'vikunni' is used here as the locative case of 'vika' (week), indicating the time period in which the action will occur.
Ég áætla að fara í sund í vikunni.
I plan to go swimming during the week.
The word 'vikunni' functions as the locative form to specify the exact timing within the week, i.e., at its end.
Við höfum mikið að gera í lok vikunnar.
We have a lot to do at the end of the week.
Here, 'vikunni' points to the duration in which the task is to be completed, implying sometime during the week.