If, in addition to the standard calendar, you would like to add further calendars, click in the “Settings” of your account on “Calendar” (on the left), then click “Add new calendar”.
You can choose a name for the calendar yourself, which can be changed later. The URL of the calendar stays the same after its creation.
New calendars can only be created through the webmail interface, not through external programs or mobile devices.
Three calendars are included with your account at no extra charge. Each additional calendar costs 0.10 EUR per month. It is possible to have as many as 20 calendars.