You can easily sync your Calendar with Google Calendar. Once synced, all the events that you add to Google will automatically appear on your Calendar.
You can easily sync your calendar, as follows:
From your Weebly Editor page, click on the calendar and open Settings.
Go to Add-ons section.
Open add-ons store.
Connect to your Google account.
From the dropdown choose the calendar and Synchronize.
By clicking on the Settings button you can Show or Hide private events.
The synced events need to be managed/deleted from Google, as they are not displayed in your Calendar settings.
Free vs Premium plans:
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 upcoming events as there are left on your event limit. Whereas, with the Premium version calendar will sync all the events.