After opening a product (in this example, an accommodation entitled "Mixed Room 6"), click on the edit icon under "Allow days for check-in":
Select the available check-in dates according to your needs and save your changes:
The frontoffice date picker will automatically be updated to only allow check-ins on these specific days. (In this example, check-ins are only allowed on Saturday or Sundays. The other days are blocked from selection).
The specific check-in day option is only available for Accommodations and Packages.