You can add a shortcut to the Calendar tab to the sidebar for quick reference in Salesforce Classic.
Note: As per the Data Access and Views: What’s Different or Not Available in Lightning Experience sidebar is not available in Lightning Experience. So although the click path below is accessible from within Lightning, changing the preference has no affect for users in Lightning.
1. Navigate to Setup.
2. Enter Activity Settings into the 'Quick Find' search box and click Activity Settings
3. Select Enable Sidebar Calendar Shortcut.
4. Click Submit.
Notes:
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