The Saxon Genitive is normally used with people but it can also be used with periods of time.
- You can take a day hike. You can take a 7 day hike.
In this case the noun "day" is being used as an adjective. Nouns as Adjectives
- You can take a day's hike. You can take a 7 days' hike. West Tours
But here we are using the Saxon Genitive to say a hike of a day or 7 days.