The manual way would be to export the Calendar files from Microsoft Outlook as comma separated values (csv) and import them into Mozilla Lightning. To do that open Microsoft Outlook and select File > Import and Export from the top menu.

The Import and Export Wizard will show up offering choices to import data into Microsoft Outlook and export data into files as well. Pick the Export To A File entry from the selection.

The Export To A File menu shows up which offers to export data into various file types. Select the Comma Separated Values (Windows) entry from the available choices.

After making that selection it is time to select the data that should be exported. Pick the Calendar entry from the available data sources.

Select a file name for the calendar export, pick the data range that you want to export and save it to the computer’s hard drive. Now that the exported data is available it is time to import it into Mozilla Lightning.

Make sure that the Mozilla Lightning extension for the email client Thunderbird has already been installed. Start Mozilla Thunderbird afterwards and click on the Calendar button in the lower left corner of the application’s window. It is possible to use the shortcut [CTRL 3] to go to the calendar as well.

Lightning supports Calendar imports from csv and ics files. Pick the csv filter in the file browser and navigate to the hard drive folder that contains the Microsoft Outlook Calendar export. A click on that file will load the calendar in Mozilla Lightning and make the calendar available there.

The Open Source tool Outpost provides an elegant alternative. It has not been updated in the last years but is still able to export Outlook data. It can be used to export Outlook Calendar data in an ics file which can then be imported into Mozilla Lightning or any other application that supports the format.