I found a workaround:

although I'd prefer to drive everything from a ViewModel I had to spread the logic in the code behind

XAML

 <igEditors:XamMonthCalendar x:Name="monthCalendar" SelectedDatesChanged="monthCalendar_SelectedDatesChanged" 

<ig:XamDayView attach:XamDayViewHelper.VisibleDates="{Binding Path=VisibleDates}" 

CODE BEHIND

            avm = new AgendaViewModel();
            DataContext = avm;

 private void monthCalendar_SelectedDatesChanged(object sender, Infragistics.Windows.Editors.Events.SelectedDatesChangedEventArgs e)
        {
            List<DateTime> tmpDates = new List<DateTime>();
            XamMonthCalendar monthCalendar = sender as XamMonthCalendar;
            if (monthCalendar != null)
            {
                switch (avm.AgendaSituation)
                {
                    case AgendaSituation.SingleOperator:
                        monthCalendar.SelectionType = SelectionType.Extended;
                        break;
                    case AgendaSituation.MultipleOperator:
                    case AgendaSituation.DentalChair:
                    default:
                        monthCalendar.SelectionType = SelectionType.Single;
                        break;
                }
                foreach (var item in monthCalendar.SelectedDates)
                {
                    tmpDates.Add(item);
                }
            }
            avm.VisibleDates = tmpDates;

        } 

where VisibleDates is 
     public static class XamDayViewHelper

    {
        public static readonly DependencyProperty VisibleDatesProperty = DependencyProperty.RegisterAttached("VisibleDates", typeof(IList<DateTime>), typeof(XamDayViewHelper), new FrameworkPropertyMetadata(null, new PropertyChangedCallback(VisibleDatesChanged)));
 
        private static void VisibleDatesChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
        {
            XamDayView dv = sender as XamDayView;
 
            if (dv != null)
            {
                dv.VisibleDates.Clear();
                foreach (var day in e.NewValue as IList<DateTime>)
                {
                    dv.VisibleDates.Add(day);
                }
            }
        }
 
        public static IList<DateTime> GetVisibleDates(DependencyObject obj)
        {
            return (IList<DateTime>)obj.GetValue(VisibleDatesProperty);
        }
 
        public static void SetVisibleDates(DependencyObject obj, IList<DateTime> value)
        {
            obj.SetValue(VisibleDatesProperty, value);
        }
    }
I don't understand why you have a SelectedDatesChanged event and do not a SelectedDates bindable property that I could read from or write to using a ViewModel.
It works for my needs.
Best Regards
 Roberto Dalmonte