Mozilla Sunbird

The method bellow was tested with the Lightning extension for Thunderbird (under Windows). HDA version 6.1 (Fedora14).

To create/subscribe to a calendar in Mozilla client:

File > New Calendar
Select On the network for the location of the new calendar
Select the iCalendar format and set the network location for the new calendar. To be sure check in your Amahi settings/calendar (http://hda/setup?sub=calendars&tab=setting) the link for the existing calendar (for me it was webcal://calendar/aes.ics: use it without the file name to create a new calendar or with the file name to subscribe to an existing calendar).
From the calendar's properties you can set the frequency of syncs, color etc.

NOTES:

You can create the calendar directly from the hda interface and then connect to it from the calendar client.
To connect to the calendar server (hda) from outside the local network see: Hosting a website, VPN, and Adito.

Amahi Wiki