How to Import an Outlook Calendar Into Lightning
Lightning is a Mozilla Thunderbird add-on that integrates the Sunbird calendar application into the Thunderbird email client. The extension is designed to work in concert with the email functionality of Thunderbird, enabling users to schedule meetings, assign tasks and appointments and convert emails to scheduled events. If you are switching to Lightning from Outlook, you can convert your calendar data for use in Lightning easily.
Launch Microsoft Outlook.
Select the "File" drop-down menu and select "Save As."
Type a file name, select "iCalendar Format" as the save type and then click "Save."
Select "Whole Calendar" as the Date Range and click "OK."
Launch Mozilla Thunderbird.
Select the "File" drop-down menu and click "Import Calendar."
Click "Browse" and select the iCalendar file that was exported from Outlook and click "OK." The calendar items will be imported into Lightning.