@Elke5
You can set check-in/out days in general for your listing in the availabilites section of the listing calender.
But if you need such a setting for a specific time period, you need to create a rule for it and apply that rule to that time period
Details about rule-sets are in this article:
https://www.airbnb.com/help/article/2062
One of the options in a rule is to set the check-in/check-out day(s)
This is an example (in Dutch, sorry...) , guest can NOT check-in on a Saturday or Sunday, but can check-out on every day.
