Why doesn't my code work?
You can't trust JsFiddle when it comes to running Javascript in Drupal, because Drupal uses the Immediately Invoked Function Expression (IIFE), a well known pattern in JS. Raw code you supply the fiddle with may not always work in the application.

See managing JS in Drupal. Your code needs to be wrapped first. My guess? You have console errors regarding $ is undefined.

(function ($, Drupal) {
  Drupal.behaviors.myCustomBehavior = {
    attach: function (context, settings) {
      $(".cal-full").mouseover(function() {

        $(this).siblings('.cal-events-tooltip').addClass('cal-events-tooltip_show');
    $(this).siblings('.cal-events-tooltip').removeClass('cal-events-tooltip_hide');

    });

$(".cal-full").mouseout(function() {

        $(this).siblings('.cal-events-tooltip').removeClass('cal-events-tooltip_show');
    $(this).siblings('.cal-events-tooltip').addClass('cal-events-tooltip_hide');

    });
    });
    }
  };
})(jQuery, Drupal);


If this is Drupal 8, you will also need to require jQuery and related dependencies from your module or theme, so they are attached to the page before your script.