How To give notifications on android on specific time?

first you need to use a broadcastreceiver. and because a broadcast receiver up only for a short time

from the android developer blog.When handling a broadcast, the application is given a fixed set of time (currently 10 seconds) in which to do its work. If it doesn’t complete in that time, the application is considered to be misbehaving, and its process immediately tossed into the background state to be killed for memory if needed.

its a better practice to use also intent service here you have a example how to do it.

this is the broadcast receiver class.

public class MyReceiver extends BroadcastReceiver {
    public MyReceiver() {
    }

    @Override
    public void onReceive(Context context, Intent intent) {

        Intent intent1 = new Intent(context, MyNewIntentService.class);
        context.startService(intent1);
    }
}

and register it in the manifest.

<receiver
    android:name=".MyReceiver"
    android:enabled="true"
    android:exported="false" >
</receiver>

this is the intent service class.

public class MyNewIntentService extends IntentService {
    private static final int NOTIFICATION_ID = 3;

    public MyNewIntentService() {
        super("MyNewIntentService");
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        Notification.Builder builder = new Notification.Builder(this);
            builder.setContentTitle("My Title");
            builder.setContentText("This is the Body");
            builder.setSmallIcon(R.drawable.whatever);
        Intent notifyIntent = new Intent(this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 2, notifyIntent, PendingIntent.FLAG_UPDATE_CURRENT);
        //to be able to launch your activity from the notification 
        builder.setContentIntent(pendingIntent);
        Notification notificationCompat = builder.build();
        NotificationManagerCompat managerCompat = NotificationManagerCompat.from(this);
        managerCompat.notify(NOTIFICATION_ID, notificationCompat);
    }
}

and register it in the manifest.

<service
    android:name=".MyNewIntentService"
    android:exported="false" >
</service>

and then in your activity set the alarm manger to start the broadcast receiver at a specific time and use AlarmManager setRepeating method to repeat it this example bellow will repeat it every day.

 Intent notifyIntent = new Intent(this,MyReceiver.class);
PendingIntent pendingIntent = PendingIntent.getBroadcast
            (context, NOTIFICATION_REMINDER_NIGHT, notifyIntent, PendingIntent.FLAG_UPDATE_CURRENT);
AlarmManager alarmManager = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
alarmManager.setRepeating(AlarmManager.RTC_WAKEUP,  System.currentTimeMillis(),
            1000 * 60 * 60 * 24, pendingIntent);

i hope this will help you.