There are 3 options for syncing your Yahoo! Calendar and/or your Yahoo! Contacts with iOS devices:

• Using Yahoo! in the Setup Wizard: Syncs your Yahoo! Mail, Yahoo! Calendar, and Yahoo! Contacts data with your iOS device at the same time, the most stable option.
  Note: Only available using iOS v5.0 and later.
• CalDAV: Only syncs your Yahoo! Calendar data, not as stable.
• CardDAV: Only syncs your Yahoo! Contacts data, not as stable.

Each option syncs bi-directionally, meaning your Yahoo! Calendar and Yahoo! Contacts data will be updated online whenever you make a change on your mobile device, and vice versa. 

Syncing using Yahoo! in the Setup Wizard:

1. From your device's Home screen, tap the Settings icon.
2. Tap Mail, Contacts, Calendars.
3. Tap Add Account.
4. Tap Yahoo!.
5. Enter your name, email address, password, and account description.
6. Tap Next.
7. Once your account has been verified, tap Save.

Syncing your Yahoo! Calendar using CalDav:

1. From your device's Home screen, tap the Settings icon.
2. Tap Mail, Contacts, Calendars.
3. Tap Add Account.
4. Tap Other.
5. Under "Calendars," tap Add CalDAV Account.
6. Enter your server (https://caldav.calendar.yahoo.com), Yahoo! ID, password, and account description.
7. Tap Next to complete set up.

Syncing your Yahoo! Contacts using CardDav:

1. From your device's Home screen, tap the Settings icon.
2. Tap Mail, Contacts, Calendars.
3. Tap Add Account.
4. Tap Other.
5. Under "Contacts," tap Add CardDAV Account.
6. Enter your server (https://carddav.address.yahoo.com), Yahoo! ID, password, and account description.
7. Tap Next to complete set up.

If you recently upgraded your device's iOS version from v4.0 or earlier, we recommend that you remove any existing CardDAV or CalDAV accounts from the device before setting up syncing. If you leave these on your device, they may cause problems with syncing.

To remove existing Yahoo! CardDAV or CalDAV accounts:

1. From your device's Home screen, tap the Settings icon.
2. Tap Mail, Contacts, Calendars.
3. Tap either Yahoo! - Contacts or Yahoo! - Calendar.
4. Tap Delete Account.
5. Tap Delete Account again to confirm deletion.