While this question needs work, (show us your code, what have you tried, what exactly do you want to do with specifics, ect), my answer would be this:
1) Display the description, but apply display: none via CSS to it’s container div to hide it. Then write some javascript for an onclick event to apply display: none to all the descriptions, but remove display: none for the event the user clicked.
OR
2) Include the post (event) ID somewhere as an html ID or a hidden input, then write some javascript onclick event to send this ID to a PHP function via ajax. Have this PHP get the description for that ID, then you place this where ever you want with javascript. This solution requires more work and an understanding of ajax though.