How to backup the Android calendar file (is there such a file)? Does anybody know which files I need to save in order to have a simple backup? 

Reason for my question is, that my Galaxy Android smartphone lost all calendar data without any interaction. I found tons of apps, tons of sync stuff, but NO simple list of files to save. I do NOT want to save my data remotely on Google.

There must be at least one local file (database?) on my phone containing the synced calendar file (on my server, using ICSSync)
Android stores calender data (and contacts as well) into a SQLite database in the system. The format of that SQLite database differs between ROMs -- so at least for "normal users" it's not that good an idea to "simply" copy that SQLite file. Rather use a good backup app for this. A good example would be iCal Import/Export, which allows you to export your calendar to an .ics file -- and also to import from .ics files, e.g. for a restore.