Loading Dates

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Microsoft.Phone.Controls;
using System.IO.IsolatedStorage;
using System.Collections.ObjectModel;
using System.Xml.Linq;

namespace LasnaoloAppsi
{
    public partial class LasnaoloAjat : PhoneApplicationPage
    {
        XDocument lista = XDocument.Load("NykyisetKurssit.xml");

        private ObservableCollection<Kurssit> _Lasnaolot = new ObservableCollection<Kurssit>();

        const string id1 = "504T11";
        const string id2 = "504T12";
        const string id3 = "504T10";
        const string id4 = "504T09";

        string d504T11 = "CurrentDate504T110";
        string d504T12 = "CurrentDate504T120";
        string d504T10 = "CurrentDate504T100";
        string d504T09 = "CurrentDate504T090";

        string id = string.Empty;

        public LasnaoloAjat()
        {
            InitializeComponent();
        }

        protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
        {
            if (NavigationContext.QueryString.TryGetValue("id", out id))
            {
                var ryhma = (from ryhmaInfo in lista.Descendants("Kurssi")
                             where ryhmaInfo.Attribute("id").Value == id
                             select new Kurssit
                             {
                                 RyhmanNimi = (string)ryhmaInfo.Element("tunnus").Value

                             }).FirstOrDefault();

                PageTitle.Text = ryhma.RyhmanNimi;

                if (LasnaoloAikojenLista.ItemsSource != null)
                {
                    return;
                }

                switch (id)
                {
                    case id1:

                        LoadLasnaoloDate(d504T11);
                        break;

                    case id2:

                        LoadLasnaoloDate(d504T12);
                        break;

                    case id3:

                        LoadLasnaoloDate(d504T10);
                        break;

                    case id4:

                        LoadLasnaoloDate(d504T09);
                        break;

                }
            }

            base.OnNavigatedTo(e);
        }

        public void LoadLasnaoloDate(string currentDate)
        {
            var settings = IsolatedStorageSettings.ApplicationSettings;

            Kurssit date;

            if (settings.TryGetValue<Kurssit>(currentDate, out date))
            {
                    foreach (string s in settings.Keys)
                        _Lasnaolot.Add(date);

                    LasnaoloAikojenLista.ItemsSource = _Lasnaolot;

            }
        }

        private void LasnaoloAika_HBTN_Click(object sender, RoutedEventArgs e)
        {
            var button2 = (HyperlinkButton)sender;
            var selectedApp2 = (Kurssit)button2.Tag;

            NavigationService.Navigate(new Uri("/LasnaoloLista.xaml?id=" + id, UriKind.Relative));
        }
    }
}