Sync is done on a PC as of now. Yahoo's direction on this is a mess. They wax and wane on direction and apps. Here I ditched the Yahoo calendar due to their waffles and their new owners.
Yahoo's Calendar uses the CalDAV internet standard and iCal (ICS) file format to allow access to your calendar (from Yahoo web site). Elsewhere Yahoo states specifically that the Android phone Calendar app doesn't support CalDAV/iCal (ICS) so it will not synch with the Yahoo Calendar but that there are Calendar Apps available from the Google Play Store that will synch with Yahoo's Calendar.
So far, I cannot find one.
Does anyone have a Calendar app running on Android that synchs with Yahoo's Calendar?