When I was having system problems, I wiped 21.1 Cinnamon and installed LMDE 6. It took a day, but I got everything installed and I was chugging along. I used Korganizer for my calendar program (I love the UI and the ability to easily change what prints). Bringing my .ics files back in was a breeze.
About two days later, the universe decided to kick me in the nads with steel toed boots. A day after making some changes in the calendars, I opened up the software to work with my bills calendar. It was blank
My heart skipped three or four beats. 
I tried deleting the calendar from Korganizer and re-installing from the .ics file, still blank. I imported the .ics into the preinstalled calendar program, still blank.
Since the .ics was originally created with thunderbird, I attempted to bring it back in. I got an error message saying there was nothing available to import.
Korganizer said the bills calendar had 0 bytes. The file Manager showed the .ics file had close to 150k in size.
At this point I knew ALL my calendar files were FUBAR!
I pulled out my backup drive and started checking timestamps. I ended up in the versioning folder I had set up. I found all my calendars from the 15th (at the time of this writing, 5 days ago). I was able to import those calendar files in Evolution without any problems. It took me a couple hours to go in and add the changes to our bills and nursing calendars. The new events for the notifications calendar is gone forever, because I don't remember what they were.
Thank you FreeFileSync with versioning. This has been a real world exercise in Backup, Backup. Backup! I did learn two thing from this event.
1. As long as you're running your backups, the versioning feature in FreeFileSync can save your bacon. Also be sure whatever backup solution you choose has a versions feature for the files. My supposed backup was also corrupted, thus forcing me into the versions. If the backup solution merely mirrored the source, then I would have been screwed (thinking basic parameters for grsync).
2: Running backups once a week isn't enough. I really wish I could remember what was added to the reminders calendar.
I also use FreeFileSync (FFS) to back up my data. I run it every evening before sacking out for the night (unless I've made no changes to data during the day). Since FFS only involves files that have been added, changed or deleted since the previous update, updates usually take only a handful of minutes to run, despite having roughly 16TB of data, including connecting and putting away the backup drives (I have four data drives in my computer so I have four backup drives).
). I get around that by, every time I add, change or delete a calendar entry, I backup the calendar entries, using Evolution's own backup function, which allows me to save the backups in a place of my own choosing, which is on a data drive that gets backed up with FFS. It takes only a few seconds.