Firstly, here's a quick video showing you how to use the Calendar App:
Secondly, here's a training video that covers the app in lots of detail (20 mins):
How to use the Calendar app:
To create an event you'll first need to go into the ticket you'd like to add it to, then in the apps pane on the right, you'll see the options to create an event. Choose the time & date, then set the duration.
The event will automatically appear below in context with other events that will happen within a day either side like this:
To view all of the events taking place that month click on the calendar icon on the left of the screen.
To manage your calendars, at the top right of the same view click on "Manage calendars"...
Here you'll be able to add, remove and control who has access to the calendars that you create...
At the top right of the "Manage Calendars" page you'll see a couple of icons. The first is a ticket which will show you a list of placeholders that you can use to reference event details in macros, triggers and automations. We've written a whole other article on this here.
The second icon is the cog linking you to the settings screen where amongst other things you'll be able to predefine what Zendesk ticket information automatically flows into your calendar event details. For more information on how this works see this article.