During my migration from Mac OS X to Linux, I faced the problem of having hundreds of events in my iCal (Apple's calendar software) that I wanted to import to Linux.

Using Thunderbird as email program in Linux, I stumbled across Lightning, a calendar add-on for Thunderbird which is capable of reading / importing files in the iCalendar format. After downloading the add-on from the Lightning website, it can be installed by clicking Tools > Add-ons > Install Add-on From File… in Thunderbird:

The next step is go to your Mac and export all calendars - one after each other - in iCal by selecting a calendar from the list and clicking Export… > Export…. The calendar will be saved as an .ics file. (Note: Doing this once in a while to make regular backups of your calendars before your Mac dies is a very good idea. I used the export option to create calendar backups for many years now, and just now it saved me a lot of trouble.)

To import iCal's backup files in Lightning, go to the Calendar tab in Thunderbird and choose Events and Tasks > Import… from the Menu:

You are prompted to pick an existing Lightning calendar to which the events from the imported file shall be added. Select one, et voilà: iCal events on your Linux!