(12,8) The Abudraham asks this question and answers from Rav Yosef Kimchi, that Hashem who knows the future commanded them before it happened to do so. This would seem to me to be along the lines of Avraham Avinu making matzah on erev pesach when "the palit" came to tell him that Lot was captured. It's a remembrance of the future. Which isn't as paradoxical as it sounds, for if you think you are going to meet someone or go somewhere special you get dressed up to do so. You don't know for sure that it will happen but you are willing to do something in case it does. If Hashem, who knows what will happen tells you to do something because of what will be, surely it's logical to do so.