Well, it's pretty obvious what a Calendar is!
Click on the hamburger menu, and choose Events (or Calendar).
There, you have different options:
Click on Create Event.
You then have several options. At the very least, you need to provide:
But you have several other options, and even more if you click on Advanced options.
You can also, like always with Hubzilla, decide who is going to see your event by clicking on the locker.
Submit when you're done.
Your event now appears in you calendar:
If you click on the even, a window pops-up. You then get more details if provided by the event creator. You can also edit (click on the pencil) or delete (click on the trash) your event if you're its creator.
Automatically, a post is created to inform your audience of the event. You can see it on your channel.
You can give a lot of information about your event, for example:
People may want to inform you that they will participate into your meeting. To do so, they just click on the calendar icon in your event post, and then click on the right answer.
As you can see in the Disroot event post above, 6 people indicated that they would attend the event.