You can import iCalendars (iCal) calendars that are saved in the .ics format. iCalendars are the standard for calendar data exchange over the Internet, and .ics is the standard format for iCalendar information. Import is available from your Preferences Import/Export tab.
You can import your iCal calendar to an existing calendar or create a new calendar and import the .ics file to it.
Go to Preferences>Import/Export.
In the Import section File field, browse to locate the calendar .ics file to import.
Select the destination for this file. Select an existing calendar or create a calendar to import the file. Click OK.
Click Import.
Click OK.
iCal calendars are updated periodically according to the remote calendar polling intervals set by the system administrator.