Risolto!


Questi sono i miei due datepicker:
codice:
<DatePicker Height="25" HorizontalAlignment="Left" Margin="3,12,0,0" Name="fromDatePicker" VerticalAlignment="Top" Width="115" SelectedDateChanged="fromDatePicker_SelectedDateChanged" />


<DatePicker Height="25" HorizontalAlignment="Left" Margin="-115,45,0,0" Name="toDatePicker" VerticalAlignment="Top" Width="115" />

qui la funziona che scatta sul change:
codice:
        private void fromDatePicker_SelectedDateChanged(object sender, SelectionChangedEventArgs e)
        {
            var fromPicker = sender as DatePicker;


            DateTime? data = fromPicker.SelectedDate;


            if (data != null)
            {
                toDatePicker.SelectedDate = data;
                toDatePicker.DisplayDateStart = data;
            }


        }

Grazie!