How to show snippet of an event when clicked in the calendar in Drupal 7 with the Calendar Module
There are probably many different ways of doing it. 

One possibility is the Calendar Tooltips module. 

Another is using Colorbox module to control the popup, you construct the Views output yourself so that the Colorbox library displays it as a popup: Does Calendar Popup Module exist in D7?, using-colorbox-display-event-details-popup