A simple guide to using Yahoo Calendar with your Thunderbird email client and having the ability so syncronise events from either side.
Follow these steps to add your Yahoo calendar to Thunderbird.
Make sure you create a separate app password from within the Yahoo Calendar. This will be used to assign to the tbsync Thunderbird addin later.
Add the following addins to Thunderbird:
Go to Tools->Syncronization Settings(tbsync)
Select account actions->Add new account->CalDAV &CardDAV
Scroll to the bottom and select Yahoo
Add your username, normally email address someone@yahoo.co.uk and the password from step one. It’ll probably be 4 sets of characters.
Click Next and the calendar should be available as read and write.