MainActivity.java

    @Override
    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
        if (key.equals("plan_length")) {
            planLength = Integer.parseInt(sharedPreferences.getString(key, getString(R.string.plan_length_default_value)));
        } else if (key.equals("first_day")) {
            firstDayOfWeek = Integer.parseInt(sharedPreferences.getString(key, getString(R.string.first_day_default_value)));
        }
        recreate();
    }