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As the map owner, you will find the “Import calendar (ics)” button on the right below the calendar. By clicking the button, you can upload an ics file. All events will then be imported automatically.

“ics” stands for "Internet Calendaring and Scheduling Core Object Specification (iCalendar)". It is a file format for sharing calendar information. With almost any external application, you can export the calendar as an ics file.