For Outlook calendar to Lightning, export the Outlook calendar info as a csv file and import the csv file into Lightning.
In Outlook, select File > Import and Export from the File menu.
In the Wizard, select the "Export to a file" option and then click Next.
In the next window, select the "Comma Separated Values (Windows)" option and then click Next.
In the next window, select the "Calendar" option and then click Next and save the file.
Copy the file to your Linux Mint box. Open Thunderbird and then Lightning.
In Lightning, select File > Import Calendar.
Select the csv file you copied over and Lightning will copy everything over.
This worked for me, but doesn't for everyone. Outlook doesn't always play well with others. I am transferring everyone at work over to Thunderbird/Lightning and only about half of the conversions are working. I'm not starting a MS bashing thread here because it doesn't help anyone. Just stating that it might not work, and if it doesn't work for you, you may need to look at other options.
There is an open source tool called Outpost that some people have had good luck with. It seems it is no longer maintained, but still works. Also, there is another open source tool called freemical that is used to transfer calendar info to Lightning. I haven't used either of these tools so I can't vouch for their success, but passing them along.
Hope this helps, good luck!