This is the 118th most frequent Finnish word.
sunnuntaina
"On Sunday."
The word 'sunnuntaina' is used here to specify the day, indicating 'on Sunday'.
Menemme mökille sunnuntaina.
We are going to the cabin on Sunday.
The word 'sunnuntaina' specifies the time of the visit, pointing to 'on Sunday'.
Ystäväni tulee käymään muutenkin sunnuntaina.
My friend is visiting on Sunday anyway.
The word 'sunnuntaina' here specifies a day within the future context, referring to 'next Sunday'.