You can add multiple accounts to Calendar, including iCloud, Exchange, Google, Facebook, Yahoo!, and CalDAV. You can view, add, and modify events in all the accounts you add. When you add or modify events, or accept invitations on one device, all your other devices that use the same account are automatically updated.
All calendars associated with an account are in the calendar list on the left. If you don’t see the calendar list, choose View > Show Calendar List.
Choose Calendar > Add Account, select the account type, then follow the onscreen instructions.
For more information, see Add an account.
Adding your iCloud account is easier in iCloud preferences. For instructions, see Set up iCloud on your Mac. Adding a second iCloud account is easier in Internet Account preferences. For instructions, see Add your Twitter, Facebook, and other Internet Accounts.
If you’re already signed in to iCloud on your Mac, you don’t need to add your iCloud account as described above. However, you may need to turn on the iCloud Calendar feature. If you don’t have an iCloud account, you can sign up for one for free when you set up iCloud on your Mac.
By default, your account is turned on when you add it. If you turn off an account, the account’s calendars no longer appear.
Choose Calendar > Preferences, then click Accounts.
Select an account, then select or deselect “Enable this account.”
Choose Calendar > Preferences, then click Accounts.
Select the calendar account, then click the Remove button 
.
If you delete an iCloud calendar account, the Calendars feature is turned off in iCloud preferences (or in Internet Account preferences if your Mac has more than one iCloud account set up). If you turn on the Calendars feature again in iCloud preferences (or Internet Account preferences), your iCloud calendars and events reappear in Calendar, up to date.
The calendars and events you see for an account you add on your Mac are the same as the calendars and events you see on your other devices that use the same account. When you add or change events, accept invitations, or make other changes in an account on one device, the changes appear automatically on your other devices that use the same account.