If appropriate, you can export items from a Calendar portlet instance into an Excel CSV file. You can export items from any Calendar portlet instance that you have permission to view.
Further, when working in your My Calendar, you have the ability to import calendar items from a CSV file. (Depending on your permissions, you may also be able to import into other Calendar portlet instances.) Note that the portlet instance into which you are importing items must already host at least one calendar.