How to save in a Preference the different data of a checkbox of a ListView

I have a custom ListView with a Text and a checkbox. The listView show all the days of the week. I'm trying to save this days in the preferences, to use this data in another screen. But I can't do it. I've read some other similars questions here but not work for me.

I must take into account that the user can save one day, four days, or all that he want. This code is working....

Thanks in advance

MainClass:

public class PlayAllDay extends Activity implements CompoundButton.OnCheckedChangeListener{

 protected  ListView lv;
    protected ArrayList<Day> dayList;
    protected HmPlayAllDayAdapter dayAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_greetings_settings_play_all_day);

        lv = (ListView) findViewById(R.id.listView);
        displayDayList();

    }

    public void displayDayList(){
        dayList = new ArrayList<Day>();
        dayList.add(new Day("Monday"));
        dayList.add(new Day("Tuesday"));
        dayList.add(new Day("Wednesday"));
        dayList.add(new Day("Thursday"));
        dayList.add(new Day("Friday"));
        dayList.add(new Day("Saturday"));
        dayList.add(new Day("Sunday"));

        dayAdapter = new PlayAllDayAdapter(dayList, this);
        lv.setAdapter(dayAdapter);
    }

    @Override
    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked){

        int position = lv.getPositionForView(buttonView);
        if(position != ListView.INVALID_POSITION){
            Day d = dayList.get(position);
            d.setSelected(isChecked);

            Toast.makeText(this, "Clicked on Day: " + d.getName(), Toast.LENGTH_SHORT).show();
        }
    }
}

}

MyAdapter class.

class Day{
    protected String name;
    protected boolean selected = false;

    public Day(String name){
        super();
        this.name = name;
    }

    public String getName(){
        return name;
    }

    public void setName(String name){
        this.name = name;
    }

    public boolean isSelected(){
        return selected;
    }

    public void setSelected(boolean selected){
        this.selected = selected;
    }
}

public class PlayAllDayAdapter extends ArrayAdapter<Day> {

    private List<Day> dayList;
    private Context context;

    public HmPlayAllDayAdapter(List<Day> dayList, Context context) {
        super(context, R.layout.main_greetings_settings_list_days, dayList);
        this.dayList = dayList;
        this.context = context;
    }

    public static class DayHolder {
        public TextView dayName;
        public CheckBox chekBoxDay;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        View v = convertView;
        DayHolder holder = new DayHolder();

        if (convertView == null) {
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = inflater.inflate(R.layout.main_greetings_settings_list_days, null);

            holder.dayName = (TextView) v.findViewById(R.id.day);
            holder.chekBoxDay = (CheckBox) v.findViewById(R.id.checkbox_day);

            holder.chekBoxDay.setOnCheckedChangeListener((HmPlayAllDay) context);
        } else {
            holder = (DayHolder) v.getTag();
        }

        Day d = dayList.get(position);
        holder.dayName.setText(d.getName());
        holder.chekBoxDay.setChecked(d.isSelected());
        holder.chekBoxDay.setTag(d);

        return v;
    }
}