I use Yahoo Calendar heavily and there is no access to it via iPhone or Ipad unless you go to the full version on Safari. The problem with the full version is that the pop-up windows that present themselves when adding a new calendar event don't work well and don't allow you to enter all the necessary data.
You used to have a mobile version of Calendar that was easy to use from an iPhone. Bring it back! Or make an app dedicated solely to Calendar!
G shared this idea