Not sure if I understand what you're asking.
Are you writing an app that needs to access a user's calendar data, presumably on the Web somewhere?
Yahoo: search the Web for that. Does anyone actually use Yahoo calendars?
Apple: Use the EventKit framework. Events (ie, calendar entries) that you add to the user's local calendar automatically get pushed to any CalDAV and Exchange calendars on the user's other devices. Meaning you don't "download" anything.
EventKit headers have been parsed for Pascal and are available here:
https://github.com/genericptr/Others? Like what?
-Phil