The article & Photo Taken source from : http://krypted.com/mac-os-x-server/configure-the-calendar-server-in-el-capitan-server/
Configuring Calendar Server in OS X Server 5 (running on El Capitan or Yosemite)
is a fairly simple and straight forward process.
The Calendar Server is a CalDAV Server, leveraging HTTP and HTTPS,
running on ports 8008 and 8443 respectively.
To enable the Calendar service in OS X Server (Server 5),
open the Server application and click on Calendar in the SERVICES section of the sidebar.
Once open, click on Enable invitations by email to enable email notifications of invitations in the Calendar Server. Provide the email address and then click on the Next button.
At the Configure Server Email Address screen, provide the type of incoming mail service in use, provide the address of the mail server and then the port number used, if not a standard port for HTTPS-based IMAP (or POP if you’d prefer), the user name and the valid password for the account. Then click on the Next button.
At the outgoing mail server screen, provide the Outgoing Mail Server address, the port, whether or not SSL is in use (it should be if possible), the password protocol, the user name and the password. Then click on the Next button.
At the Mail Account Summary screen, review the settings and if correct, click Finish. Back at the service configuration screen, click on the plus sign (“+”) and provide a type of location, an address, a delegate, a name for the location, whether or not invitations to the resource are accepted and then enter the account name for any accounts that can manage the location’s calendar (they will auto-complete, so there’s no need to remember users and groups exactly). Click Done to complete the setup. Use the Resource setting in type to configure a resource instead of a location. The two are the same, except the Type field.
CalDAV from the Account Type menu and then enter the User Name and password configured on the server, and add the address of the server if you don’t have any service records pointing to the server. The User Name is usually the name provided in Server app, followed by @ and then the address of the server.
Once the server is configured it appears in the list of accounts in the sidebar of the Calendar app. Create calendars in the account and then to share a calendar, right-click on the calendar and click on Share Calendar…
At the Share Calendar screen, provide the name the calendar should appear as to others and anyone with whom you’d like to share your calendar with.Click on the Delegation tab to view any accounts you’ve been given access to.