It is located in Case List View. So, you can create a List View based on criteria's fit your requirement and mass close case there by select case you or your user want to close them. Here is the step:
Create a List View in Case
Close button should be visible by default.
If for some reason you not see the button in List View, go to Setup | Customize | Cases | Search Layouts | Cases List View and click Edit link.
Make sure Close [MassClose] is checked.
So, when user click Close button in case list view, a new window will be open for the new Status, Case Reason and Internal Comments (not mandatory), but I do not see if possible to customize this mass Close Case screen.