Lightning is a great calendar plug-in for Mozilla's Thunderbird email client. If you are importing your existing calendar from another program, there's something to watch out for!

You can import either an iCalendar (ics) or Outlook comma delimited (csv) file into Lightning, but if you use the CSV format, you need to change the date format to four-digit years before importing. You can do that by opening the csv file with Notepad (or some other text editor) and using the Find & Replace feature to find /09" and replace it with /2009". If you don't make that change before running the import, all of your appointments will show up in the year 1909 instead of 2009!

After the import, you should verify that all of your recurring appointments are still scheduled to recur.