All Calendars, Contacts & Notes can be synchronized.
See https://weekcalendar.zendesk.com/entries/28862002 for general setup.
It's only possible to sync Yahoo Calendar events from the last 6 months, and those up to one year in the future. Events outside of these time frames won't be synced to your device (see https://help.yahoo.com/kb/SLN14840.html#cont5 for details)
Do not use CalDav synchronization in combination with Yahoo, this can result in random deletion of events.
If you use the built-in Yahoo option via System Settings for setting up a Yahoo account there shouldn't be any problems!
Please see https://weekcalendar.zendesk.com/entries/23274166 for details about this subject