CalDAV is a calendar syncing protocol, which simply means that a handful of calendars (like Apple iCal, Mozilla Lightning/Sunbird) use it to make their data interchangeable across other calendars (such as Yahoo! Calendar).

The Yahoo! Calendar server URL is:

https://caldav.calendar.yahoo.com (The 's' at the end of https is very important). You can use this URL to sync your other CalDAV-enabled calendar with Yahoo! Calendar.