You can publish sub-calendars for other people.
Click on Publish... (Veröffentlichen...) and after right-click on the sub-calendar.
In the dialogue box you have to activate the checkbox near Publish this calendar (Diesen Kalender veröffentlichen). Optionally you can include appointments marked as private and define the time span.
The Publish Location (Veröffentlichungsort) shows the unique URL of your calendar.