Although there is no Preview button, you can publish the event privately so that the event is not published to the event calendar.  Only those with the specific private URL can get to the event to review the content and look. Once the event needs to go live, you can publish the event publicly. 

To publish the event:
  1. Select Content > Events
  2. Click Edit on the event
  3. Click Publish
  4. Select either radio buttons for Published (Public) or Published (Private)
  5. Click Finish