First, to understand this task, let's compare how a description renders in a Calendar event versus, say, a Maniphest task. Create a Maniphest task with a description of something like:

```Some test description```

After saving the Maniphest task, you'll see the description render in the detail view as:

Some test description

If you repeat the same exercise with a Calendar event, this would not be true. The Calendar event detail view will show the saved description as:

```Some test description```

If you look at the PhabricatorCalendarEventEditController, the description control is added on line 440. Currently, the control is a AphrontFormTextAreaControl. It should be a PhabricatorRemarkupControl. Remarkup allows users to style their text and add meaningful text decorations. After you convert the description to the right control, the control in the edit view will now have a little menu along the top of the control that offers quick inserts of standard remarkup keys. However, if you save the description now, you will see that the description is still not rendered correctly in the detail view.
That view is built in PhabricatorCalendarEventViewController in the buildPropertyView method. You can use the same method in a similar file in the Spaces application for an example of this. After you implement that, refreshing that Calendar event you created should now render the description with remarkup. Let me know if I can clarify anything.