To sync your Calendar with Outlook Calendar please follow these steps:
Double click on the calendar to open the Settings.
Click on the Add-ons section.
Open Add-ons Store.
Add To Site the Outlook add-on.
Once Outlook add-on is added, click on Outlook from Add-ons section.
Choose Outlook and allow the access.
From the dropdown menu choose the calendar you want to sync with.
Click on Synchronize.
You can adjust the time range for synced events by going to the Settings section of this add-on.
You can sync your calendar both in Free and Premium plans. But note that in the Free version of the Calendar only 10 events can be displayed. So the calendar will sync only as many events as there are left on your event limit. Whereas with the Premium versions calendar will sync all the events.
All the events that are synced from the external calendar are not available in our Calendar settings. So, you should modify your synced events only in Outlook Calendar settings.