"Speak to the whole congregation of Israel": The speaking was on Rosh Chodesh; the taking (of the lamb), on the tenth; and the slaughtering, on the fourteenth. You say this, but perhaps the speaking and the taking were on the tenth and the slaughtering on the fourteenth? It is, therefore, written "This month is for you. Speak, etc." When was the speaking? On Rosh Chodesh. You must revert, then, to the first formulation. The speaking was on Rosh Chodesh, the taking on the tenth, and the slaughtering on the fourteenth.