// The client event 'VisibleMonthChanged' takes two parameters sender and e
// sender is the object which is raising the event
// e is theVisibleMonthChangedEventArgs
function WebMonthCalendar1_VisibleMonthChanged(sender, e) {
//Gets the new date
var newDate = e.get_date();
//Gets the old date
var oldDate = e.get_oldDate();
//Gets reference to the WebMonthCalendar
var calendar = $find("WebMonthCalendar1");
//Gets the date of the first day of WebMonthCalendar
var visibleMonth = calendar.get_visibleMonth();
}