Как настроить уведомление, которое покажет, когда какое-то событие близко.

Maybe this can clear it up a bit: http://blog.blundell-apps.com/notification-for-a-user-chosen-time/

Если вы хотите получить уведомление за день до данных todo, вы просто минус 1 день с даты.

1) Установите будильник

 getSystemService(Context.ALARM_SERVICE).set(AlarmManager.RTC, date.getTimeInMillis(), pendingIntent);

2) Catch Alarm

3) Показать уведомление

       //This is the 'title' of the notification
        CharSequence title = "Alarm!!";
       //This is the icon to use on the notification
        int icon = R.drawable.ic_dialog_alert;
       //This is the scrolling text of the notification
        CharSequence text = "Your notification time is upon us.";       
       //What time to show on the notification
        long time = System.currentTimeMillis();

        Notification notification = new Notification(icon, text, time);

       //The PendingIntent to launch our activity if the user selects this notification
        PendingIntent contentIntent = PendingIntent.getActivity(this, 0, new Intent(this, SecondActivity.class), 0);

       //Set the info for the views that show in the notification panel.
        notification.setLatestEventInfo(this, title, text, contentIntent);

       //Clear the notification when it is pressed
        notification.flags |= Notification.FLAG_AUTO_CANCEL;

       //Send the notification to the system.
        mNM.notify(NOTIFICATION, notification);

Полный исходный код