AngularJS - ForEach not looping

This looks like an error with handling your REST response. The population of the array is happening after the return statement, as the REST call is asynchronous. Try moving the loop that isn't working into it's own function, and then calling that function inside the REST success callback, or return a Promise and resolve it in the success callback.

Returning a promise:

app.service('getUserEventsById', function ($http, $q) {  

    this.getUserEventsById = function () {
        var deferred = $q.defer();
        var holidays = [];


        $http.get("http://localhost:9847/AceTracker.svc/GetAllEventsByUser?user_id=1").then(function (data) {
            for (var key in data.GetAllEventsByUserResult) {
                if (data.GetAllEventsByUserResult.hasOwnProperty(key)) {
                    holidays.push(data.GetAllEventsByUserResult[key])
                }
            }
            deferred.resolve(holidays);
        });
        return deferred.promise;
    };
});

Looping on the promise:

holidaysnew.then(function(holidays) {
    holidays.forEach(function (hol) {
        $scope.eventSources[0].events.push({
            end: $filter('dateFilter')(hol.HOLIDAY_END),
            start: $filter('dateFilter')(hol.HOLIDAY_START),
            title: hol.HOLIDAY_TITLE,
            color: $filter('colorEventFilter')(hol.HOLIDAY_EVENT_STATE_ID)
        });
    });
});

Change service to return the $http promise but to also to do all the data manipulation you need in the service itself to create the calendar events and return those.

success is deprecated in favor of using then() promise chain callbacks

A more conventional approach would look like:

app.service('getUserEventsById', function($http, $q) {

  this.getUserEventsById = function() {

    var requestPromise = $http.get("url")
      .then(function(response) {
        var holidays = [];
        var results = response.data.GetAllEventsByUserResult
        for (var key in results) {
          if (results.hasOwnProperty(key)) {
            holidays.push(results[key])
          }
        }
        // return holidays array to next `then()`
        return holidays;
      }).then(function(holidays) {
        var holidayEvents = holidays.map(function(hol) {
          // make sure to inject `$filter` in service
          return {
            end: $filter('dateFilter')(hol.HOLIDAY_END),
            start: $filter('dateFilter')(hol.HOLIDAY_START),
            title: hol.HOLIDAY_TITLE,
            color: $filter('colorEventFilter')(hol.HOLIDAY_EVENT_STATE_ID)
          }

        });
        // return the events array to next `then()`
        return holidayEvents;

      });
    // return the `$http` promise to controller
    return requestPromise;

  };
});

Then in controller all you would need is

getUserEventsById.getUserEventsById().then(function(holidayEvents){
    $scope.eventSources[0].events = holidayEvents;
}).catch(function(){

   // do something if promise chain has a rejection
})

A better solution/approach would be to make your api call from resolve.

angular
    .module('app')
    .config(config);

function config($routeProvider) {
    $routeProvider
        .when('/', {
            templateUrl: 'yourTemp.html',
            controller: 'Main',
            controllerAs: 'vm',
            resolve: {
                getUserEventsById: function(getUserEventsById){
                    return getUserEventsById.getUserEventsById()
                }
            }
        });
}

angular
    .module('app')
    .controller('Main', Main);

Main.$inject = ['getUserEventsById'];
function Main(getUserEventsById) {
      var vm = this;
      vm.yourdata = getUserEventsById.data;
}

Now you could run your forEach loop on vm.yourdata, which would run fine.