private int monthIndex = DateTime.Today.Month - 1;

public int MonthIndex
{
    get { return monthIndex; }
    set
    {
        if (monthIndex != value)
        {
            monthIndex = value;
            RaisePropertyChanged("MonthIndex");
        }
    }
}

private int _dayIndex = DateTime.Today.Day - 1;

<ComboBox Name="cboDay"
          ItemsSource="{Binding DaysList, Mode=OneTime}"
          DisplayMemberPath="fltDay"
          SelectedIndex="{Binding DayIndex, Mode=TwoWay}"
          IsEditable="True" />

public ObservableCollection<Day> DaysList { get; set; }
private int _dayIndex;

public int DayIndex
{
    get
    {
        // perform some calculation logic;
        return _dayIndex;
    }

    set
    {
        if (_dayIndex != value)
        {
            _dayIndex = value;
            RaisePropertyChanged("DayIndex");
        }
    }
}