First you should make sure that a date field is added to your checkout. In order to do it please use the checkout customizer and add a pickup date field with the date picker enabled.
After that you can select the date field in Catalog > Settings > Shipping > Order Pickup:
After you select the correct date field you will see that the date picker in checkout will become active. Only some of the days will become available to select:
The customer will be able to select only the number of days selected in order pickup settings.