In the past, about the years I was in sixth-form and first-year university, I used KOrganizer for my calendars, which is the first calendar software that I use, in conjunction with Kontact. I love KOrganizer. It is feature-rich. It allows me to set categories on my events, recurrences with clean interface for exceptions, start time and end time on my tasks, etc.
However, as the era of mobile and cloud computing comes, it is a huge convenience for me to access information everywhere, including desktops, laptops, tablets and phones. People normally use commercial online services for this, for example, Google Calendar. I was not an exception, and put my events in my Google account. Later I moved to Microsoft services.
However, as KOrganizer does not work well with these commercial online services (some features in KOrganizer cannot be used with Google, and I still can’t find a way to have read-write access to Outlook.com calendar), I finally decided to set up my own CalDAV server using radicale on my Debian server, which is backed up automatically periodically, and set up KOrganizer to connect to it.
From now on, my calendar is completely under my control, safe and secure.