Therefor keep watch, because you do not know <b><u>on</u></b> what day your Lord will come.<br/><br/>I assume "on" in the center don't cause almost no difference in the context.. <br/><br/>Also I assume it is not meant to be used. / and on can be replaced about or omitted I recken..<br/><br/>if there's a slight difference or something you know about that, comment on it, please