I have been helping Lavan out with the issues when using this widget with SS 2.4.
Here are a list of all the changes made:
EventCalendarWidget.ss, line 1 : Change the class of the div to something other than "EventCalendarWidget". I suggest something like "Widget_EventCalendar"
EventCalendarWidget.js, line 3 : Change the jQuery selector to the same class as used in line 1 of EventCalendarWidget.ss
EventCalendarWidget.js, line 27 : Change from "location.href = this.options.calendar + dateText;" to "location.href = this.options.calendar + "view/" + dateText;"
(line numbers change heaps but should be fairly straight forward)
EventCalendarWidget.php : Change the class that EventCalendarWidget_Controller extends from Controller to Widget_Controller
EventCalendarWidget.php : Move (or copy) the Content method from the Model class (EventCalendarWidget) to the Controller class (EventCalendarWidget_Controller)
(this might be optional but should be done as best practice as a page should not have multiple jQuery libraries)
Some of these Lavan already said to do but thought I would put them together in one place.
I've tested this with SS 2.4.2 and have attached the widget with all of the above changes made to it