This is the 563rd most frequent Norwegian word.
søndagens
"Belonging to Sunday" or "Sunday's."
Here, 'søndagens' is the possessive form, modifying 'avis' to indicate that the newspaper is specific to the Sunday being referred to.
Søndagens avis inneholdt mange interessante artikler.
Sunday's newspaper contained many interesting articles.
In this case, 'søndagens' describes 'hendelser', specifying that the events in question took place on Sunday.
Vi diskuterte søndagens hendelser på møtet.
We discussed Sunday's events at the meeting.
'Søndagens' here modifies 'værmelding', indicating that the weather forecast pertains to Sunday.