How to load appointments on demand in WPF Scheduler?

private async void OnScheduleQueryAppointments(object sender, QueryAppointmentsEventArgs e)
{
    schedule.ShowBusyIndicator = true;
    await Task.Delay(1000);
    schedule.ItemsSource = this.GenerateSchedulerAppointments(e.VisibleDateRange);
    schedule.ShowBusyIndicator = false;
}
 
private IEnumerable GenerateSchedulerAppointments(DateRange dateRange)
{
    Random random = new Random();
    int daysCount = (dateRange.ActualEndDate - dateRange.ActualStartDate).Days;
    var appointments = new ObservableCollection<ScheduleAppointment>();
 
    for (int i = 0; i < 50; i++)
    {
        var startTime = dateRange.ActualStartDate.AddDays(random.Next(0, daysCount + 1)).AddHours(random.Next(0, 24));
        appointments.Add(new ScheduleAppointment
        {
            StartTime = startTime,
            EndTime = startTime.AddHours(1),
            Subject = subjectCollection[random.Next(0, subjectCollection.Count)],
            AppointmentBackground = brush[random.Next(0, brush.Count)],
        });
    }
    return appointments;
}