private void Save_Click(object sender, RoutedEventArgs e)
        {
            LessonPlanItem lesson = new LessonPlanItem();

            lesson.Lesson = Lessons.SelectedItem as Lesson;
            switch (Days.SelectedIndex)
            {
            case 0:
                lesson.Day = DayOfWeek.Monday;
                break;

            case 1:
                lesson.Day = DayOfWeek.Tuesday;
                break;

            case 2:
                lesson.Day = DayOfWeek.Wednesday;
                break;

            case 3:
                lesson.Day = DayOfWeek.Thursday;
                break;

            case 4:
                lesson.Day = DayOfWeek.Friday;
                break;
            }
            lesson.BeginDateTime = (DateTime)BeginDateTime.Value;
            lesson.EndDateTime   = (DateTime)EndDateTime.Value;
            Database.LessonPlan.Add(lesson);
            NavigationService.Navigate(new Uri("/LessonPlanPage.xaml?index=" + Days.SelectedIndex, UriKind.Relative));
        }
        private void DeleteLesson_Click(object sender, RoutedEventArgs e)
        {
            LessonPlanItem selectedItem = (sender as MenuItem).DataContext as LessonPlanItem;

            Database.LessonPlan.Remove(selectedItem);
            Database.UpdateLessonPlan();
            int index = LessonPlan.SelectedIndex;

            NavigationService.Navigate(new Uri("/LessonPlanPage.xaml?date=" + DateTime.Now + "&index=" + index, UriKind.Relative));
        }