<div ng-repeat="day in days_array">
   {{getWeek(day)}}
</div>

$scope.loadDays = function () {
    service.getDays().then(function (days_array) {
        $scope.days_array = days_array.
    });
}

$scope.days_array = [{index: 0, code: MONDAY}...];

$scope.days_array = $scope.days_array.map(function (day) {
    day.week = getWeek(day);
    return day;
});

<div ng-repeat="day in days_array">
   {{day.week}}
</div>

<div ng-repeat="day in days_array">
   {{::day.week}}
</div>

<div ng-repeat="day in ::days_array">
   {{::day.week}}
</div>