Expose filter with collabsible div on click in a Calendar
I'm going to assume that each event is a node in Drupal, yes?

The filters you are describing are called 'facets', and there are a number of ways you can get them, but probably the most robust is to:


Install Apache Solr on your server if you haven't already
Install the Solr Search module in Drupal and configure it to search your events
Install the Facets API module and enable facets for your Country attribute


All of these things are well documented.

The one I had the most trouble with was the initial install of Apache Solr, but the instructions are specific to your O/S so unless you're using RedHat or a derivative, the blog post that helped me won't be very useful.