This is the 159th most frequent Finnish word.
viikon
"Viikon" means "of the week" or "week's" in English.
In this sentence, 'viikon' is the genitive case of 'viikko', meaning 'week', used to indicate possession or relation, effectively saying 'a week's time'.
Matkustan Suomeen viikon päästä.
I will travel to Finland in a week's time.
Here, 'viikon' again is the genitive case of 'viikko', used to indicate that the plan pertains to the week.
Tämä on viikon suunnitelma.
This is the week's plan.
In this context, 'viikon' serves to specify the time frame during which the events occurred, employing the genitive case for duration.