1. You cannot preview a calendar event in the same way as tasks however you can get a snap shot of the event by entering a summary of the event in the What: part of the event.

2. In the calendar event you can add more information in the What: part of the event rather than the description. 

3. Then once you have saved this information go to the calendar events screen and click on that calendar event.

4. When you click once onto an event to edit an edit window will come up but it will display the information you have put into the What: part of the event (you will be limited on how much text displays) but it gives you enough of a preview to know what the event is as shown in the screenshot below: