Google Calendar allows individual calendars to be marked as public, meaning that they can be accessed either via the Calendar UI, embeded calendars, or the Calendar Data API without requiring a password.
Normally, this option is enabled from within the Calendar UI. However, it can be enabled using the Calendar Data API as well. To do so, you'd send the following HTTP request:
POST /calendar/feeds/default/acl/full Host: www.google.com
In this case, this will make the default calendar as public. If you want to make a secondary calendar public, replace
default in the request's location with the desired calendar ID.