Restrictng Check in days

You can't. The system doesn't allow it.

You could block out alternate weeks which forces people to book only those weeks that are not blocked, and then open up the blocked weeks when there's a booking either side. Or you could have 2 listings, each having alternate weeks.

Or you could just make life easy and use AirBnB / TripAdvisor etc etc .....

You can ask booking.com to add to your calendar button no arrival no departure so then you can restrict days.

Wish you all the best...

Yes, we have done the same, just send a message to them via your admin console and they add the options to your calendar to block check in or departure days.