This is the 150th most frequent Norwegian word.
søndag
Sunday.
Here, 'søndag' refers to 'Sunday' in general as a day of the week.
Vi pleier å spille fotball på søndag.
We usually play soccer on Sunday.
This use of 'søndag' specifies a recurring event that happens on Sundays.
Hver eneste søndag spiser vi en familielunsj.
Every single Sunday, we have a family lunch.
In this sentence, 'søndag' is used with an indefinite article to refer to an unspecified Sunday in the past.