If there is a site on your Calendar that is closed and you'd like to make bookable, start by selecting the day. When you click on the day, you should see at least one site marked "Closed":
To close an individual site, select the corresponding box to switch it to "Closed".
If you want to adjust the bookability for multiple listings and/or sites at once, click "Change all":
In your Host Dashboard under the "Calendar" tab you can change the status of a date by clicking on it and using the drop down menu to select "reservable." You can change the status for multiple days across multiple sites by clicking on one date holding down your mouse and dragging across multiple days.