There are two options:

• import from URL, which requires a Public Calendar URL in a .ics format
• upload & import, which requires a .ics file with all your calendar data

An ICS (Internet Calendar Scheduling) file or URL is a calendar file saved in a universal format used by several email and calendar programs, including Apple Calendar, Microsoft Outlook, and Google Calendar.

Learn here how to:

Import from Apple Calendar

Import from Microsoft Calendar

Import from Google Calendar