וידעת היום, "and you should know this day, etc." What is it that the people should know about on this day rather than on any other day? What knowledge were they to store up in their hearts? Perhaps Moses referred to the need to subdue the evil urge which lurks within all of us. Moses may have referred to 10 such pieces of information which the Israelites were to store up in their hearts.
1) The Talmud Berachot 5 suggests that if the evil urge becomes active our first line of action should be to "drag him to the Torah Academy," i.e. to counter him by Torah study. If this fails, we must remind him of the day we will die, an effective means of putting him in his place.
2) Sukkah 52 describes that in the future God will "slaughter" Satan in front of the eyes of all of Israel. Moses commanded the people to store up an awareness of this day in the future in their hearts. Awareness of this will be a valuable tool in the fight against the evil urge.
3) Moses also referred to the Day of Judgment. What will a person do when God presents him with a list of all his sins on that day? How profound will be his feeling of shame at that time when he will be found wanting in front of an audience of illustrious souls and angels? Reminding oneself of this scenario will help one to subdue one's sinful instincts
4) Moses may have had in mind a statement in Baba Batra 84 that the reason the sun appears so reddish looking in the morning and in the evening is that it has just passed by the gates of hell whose fires have reddened it. One look at what happens in the course of a day will convince a person that the people who violate God's commandments will experience retribution. The reddish appearance of the sun is a reminder of where this retribution is handed out. Moses advises us to concentrate on what goes on in the world by day, i.e. היום.
5) He may also have alluded to what the kabbalists say i.e. that the sphere of sanctity is known as "day," whereas the sphere of alienation (from God) is known as "night." Parts of the world which belonged to the sphere of sanctity may become "alienated" through the deeds of man so that man will find himself as belonging to that sphere. This is why Moses prayed in Psalms 91,5 that God "should remove the fear of night." When Moses said וידעת היום, "know the (sphere called) day," he meant that we should endeavour to know this sphere; it would then follow automatically that we would pursue the proper path in life.
6) He may also have referred to something we have been taught by the Zohar volume three page 178 that in order to conduct oneself properly it is necessary to review every night what one has done during the day so as to repair any spiritual damage one had become guilty of by day. When a person keeps track of his deeds on a daily basis he will repent all the inadvertently committed mistakes he has become guilty of, and he will confess them. Unless he does this, his sins are liable to create within him a lack of sensitivity to having made mistakes, even inadvertent mistakes. This is why Moses commanded the people to make sure they would become aware of their actions on a daily basis, i.e. היום, this very day; do not wait until the Day of Atonement to confess your sins and to rehabilitate yourselves. If you do this, והשבות אל לבבך, you will ensure that your heart remains conscious of and sensitive to your shortcomings.
Moses may also have alluded to the day the soul was placed in the body of a person. According to Zohar volume one page 233 on that day God exhorts the soul which has come from the celestial regions to observe the commandments of the Torah while on earth, threatening with a variety of penalties for non-observance, but also promising reward for observance. When Moses said היום in our verse, he referred to the day when the soul began its journey on earth.
8) Moses may also have alluded to something we learned in Chagigah 12 that the present material world on earth is called לילה, "night," whereas the celestial regions are known as יום, "day." When observing life in this material world man will observe that often what occurs is the very opposite of what he expected to occur, such as the wicked enjoying the good life whereas the pious frequently live a life of suffering. Moses advises us to concentrate on life in the eternal regions, i.e. in the Hereafter instead of falling victim to the lure of what the transient life on earth appears to offer the sinner. He reminds us by saying והשבת אל לבבך that God has an ironclad answer to all these apparent perversions of justice we observe in this world and that all the sinners will be punished while the pious will be rewarded in due course.
9) Moses may also have alluded to an interpretation in Shabbat 28 on Genesis 1,31: "it was evening it was morning the sixth day." According to the Talmud the letter ה before the word ששי refers to a pact made by God with His creation that in due course He would give Israel the Torah on the sixth day (of Sivan). Moses reminds the people never to forget that glorious day which they had experienced when God had revealed Himself at Mount Sinai. They were to remember that creation itself continues to exist only for the sake of that day. This must be enough to remind their hearts to be constantly aware of the awesomeness of their obligation to observe the path of Torah. The words כי ה' הוא האלוהים are a continuation of the words והשבות אל לבבך. By contemplating the day on which God revealed Himself on Mount Sinai, the whole world found out that there is no other God in heaven nor on earth. This is parallel to what we learned in the Mechilta on Exodus 20,2 that when God began the decalogue He referred to having manifested Himself publicly when He took the Israelites out of Egypt. The author of the Mechilta there quotes Isaiah 48,16 who said: לא מראש בסתר דברתי, "From the beginning I did not speak in secret."
10) Moses may also have alluded to something we learned in Shemot Rabbah 25,12 that the commandment of the Sabbath is equal in weight to all the other commandments combined. It is in the nature of the evil urge that first he undermines a Jew's desire by saying to him that it is quite futile to endeavour to keep all the commandments, he would never succeed, so why even bother? At the same time, when a Jew has become aware of his sins and he wants to rehabilitate himself Satan tells him that he deludes himself if he thinks that a person who has become guilty of so many sins has a chance to purify himself through penitence. Moses counters these two arguments of Satan. Concerning a person's inability to observe all the commandments, Moses points to the fulfillment of a single commandment, that of היום, the Sabbath day, and assures the Jew that fulfillment of the Sabbath legislation is equivalent to fulfillment of all the 613 commandments. It is not so hard therefore to be a Torah-observant Jew. The scriptural proof for the value of this commandment is found in Exodus 16,25: "for the Sabbath is the day." Moses tells us to concentrate on the meaning of that day and the fact that it is equal in weight to the combined commandments of the Torah, and that this will keep us from sinning. The same thought is also useful for the sinner who wants to become a penitent and who has been told by Satan not to waste his time. If the sinner concentrates on that day and starts by observing the Sabbath he has gone a long way towards rehabilitation. God will give him credit for having observed all the 613 commandments.