DigiCal doesn't have the option to backup your calendar events, because all events are already saved on your calendar servers such as Google Calendar or Exchange. So there is no need to back it up. Moreover, it's impossible to restore the backup due to a limitation of the Android API.
However, you can always make a backup of your calendar events through Google Calendar by exporting the events and save the file. Here's a help page explaining how to do that in Google Calendar.
To make sure that all your calendar data is stored on Google Calendar, you can go to the web interface of Google Calendar (www.google.com/calendar) to check it out.
Google is making it easier to download your Google Calendar data.