Adding Yahoo to Your iPhone
Yahoo Calendar is available on the iPhone as part of a linked Yahoo Mail account. With Yahoo Calendar activated, events appear in your iPhone calendar automatically. Setting up Yahoo Calendar and the iPhone Calendar to synchronize in the background is easy. Any changes to the Calendar updates on both the iPhone and your Yahoo account. If you have not yet added the Yahoo as an email account to iPhone Mail:
Activating Calendar Sync in Existing Yahoo Account
If you added Yahoo to your Mail account previously, you might not have turned on the Calendar feature. To check this, go to Settings > Passwords & Accounts > Yahoo and confirm the slider next to Calendars is set to On/green. If not, turn it on.
Setting Sync Frequency for Yahoo
To set up the frequency your iPhone checks for Yahoo mail and calendar additions:
Remove a Synced Yahoo Account From Your iPhone
If you find that your account isn’t syncing properly, you should delete and then re-add your Yahoo account. To remove a synced Yahoo Calendar account from your iPhone:
