God's Covenant with Israel 

God reiterated His covenant with Israel to His servant Moses. Moses called for the elders of Israel and repeated God's words to them. The elders then repeated the words to the people of Israel. This gives a clearer explanation of how Moses communicated with nearly three million people. Now we come to the point where God was planning to speak with Moses and all the people would be able to hear God's voice. But there were special instructions for the people to follow before they could approach the vicinity of God's holy presence. Boundaries were set about the mountain so the people would be restricted from touching it. The prohibition against touching the mountain was to teach them a sense of awe and respect toward the living God—and to demonstrate their need for a mediator. The people were to be clean, having their clothes washed. And on the day that God appeared to Moses on the mountain, married couples were to forego sexual relations. Wearing clean clothes and abstaining from marital relations were outward acts signifying that they had sanctified themselves before God spoke to them. This does not imply that lawful sexual relations are spiritually unclean. In the New Testament, Paul suggested that it is occasionally appropriate to refrain from marital relations for a brief time,when specially devoting that time to God through prayer and fasting (1 Corinthians 7:5 1 Corinthians 7:5Defraud you not one the other, except it be with consent for a time, that you may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
American King James Version×). After Moses ascended the mountain, God had to send him back down because curiosity was getting the better of the people. After once again warning the people, Moses again ascended the mountain with Aaron.

The timing of all of this is very interesting. Jewish tradition asserts that the giving of the law occurred on the Feast of Firstfruits or Pentecost, which can occur no later than the 10th or 11th day of the third month of the Hebrew calendar, Sivan. Verse 1 does say that it was in the third month after leaving Egypt—but some interpret the phrase "on the same day" here to mean the same day of the month that the Israelites left Egypt. This, however, would mean that they arrived at Mount Sinai on the 15th of Sivan, with the law being given on the 17th (compare verses 10-11)—too late for Pentecost. However, if the phrase "on the same day" is understood to mean the same day that Jethro departed, as stated in the previous verse (18:27), then Pentecost can fit quite well. It could also be that the "same day" meant the same day of the week the Israelites had left Egypt—which, again, would allow for the Ten Commandments to have been delivered on Pentecost.

Indeed, there are clear Pentecost themes to be found here: the consecration of Israel as the chosen people, i.e. "firstfruits"; the beginning of the Old Testament "church in the wilderness" (Acts 7:38 Acts 7:38This is he, that was in the church in the wilderness with the angel which spoke to him in the mount Sina, and with our fathers: who received the lively oracles to give to us:
American King James Version×KJV), as Pentecost would mark the beginning of the New Testament Church (see Acts 2); the giving of the law, as God's people would later be given the power to keep that law through the Holy Spirit on Pentecost (compare Luke 24:49 Luke 24:49And, behold, I send the promise of my Father on you: but tarry you in the city of Jerusalem, until you be endued with power from on high.
American King James Version×; Romans 8:7 Romans 8:7Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be.
American King James Version×); God descending on the mountain with great noise and trembling and "in fire" (Exodus 19:18 Exodus 19:18And mount Sinai was altogether on a smoke, because the LORD descended on it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.
American King James Version×), as His presence would later descend upon Christ's disciples with great noise and in tongues of fire (Acts 2); the initiation of the Old Covenant, as Pentecost would later mark the giving of the "better promises" of the New Covenant, particularly the gift of the Holy Spirit (compare Hebrews 8:6 Hebrews 8:6But now has he obtained a more excellent ministry, by how much also he is the mediator of a better covenant, which was established on better promises.
American King James Version×). Though typical of the new relationship God wants with His people, the Old Covenant still involved separation from God, as the boundary markers so vividly picture. To see this even more, read Hebrews 12:18-28 Hebrews 12:18-28 [18] For you are not come to the mount that might be touched, and that burned with fire, nor to blackness, and darkness, and tempest, [19] And the sound of a trumpet, and the voice of words; which voice they that heard entreated that the word should not be spoken to them any more: [20] (For they could not endure that which was commanded, And if so much as a beast touch the mountain, it shall be stoned, or thrust through with a dart: [21] And so terrible was the sight, that Moses said, I exceedingly fear and quake:) [22] But you are come to mount Sion, and to the city of the living God, the heavenly Jerusalem, and to an innumerable company of angels, [23] To the general assembly and church of the firstborn, which are written in heaven, and to God the Judge of all, and to the spirits of just men made perfect, [24] And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaks better things that that of Abel. [25] See that you refuse not him that speaks. For if they escaped not who refused him that spoke on earth, much more shall not we escape, if we turn away from him that speaks from heaven: [26] Whose voice then shook the earth: but now he has promised, saying, Yet once more I shake not the earth only, but also heaven. [27] And this word, Yet once more, signifies the removing of those things that are shaken, as of things that are made, that those things which cannot be shaken may remain. [28] Why we receiving a kingdom which cannot be moved, let us have grace, whereby we may serve God acceptably with reverence and godly fear:
American King James Version×.

The contrast between the Old and New Covenants is vividly illustrated by comparing two scriptures. "You shall set bounds for the people all around" (Exodus 19:12 Exodus 19:12And you shall set bounds to the people round about, saying, Take heed to yourselves, that you go not up into the mount, or touch the border of it: whoever touches the mount shall be surely put to death:
American King James Version×) and "let us draw near with a true heart in full assurance of faith" (Hebrews 10:22 Hebrews 10:22Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.
American King James Version×). Through Jesus Christ's sacrifice and intercession as our High Priest today, God has granted us liberty to come right before His very throne of grace (Hebrews 4:14-16 Hebrews 4:14-16 [14] Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession. [15] For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin. [16] Let us therefore come boldly to the throne of grace, that we may obtain mercy, and find grace to help in time of need.
American King James Version×).

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Supplementary Reading: "The Feast of Pentecost: The Firstfruits of God's Harvest", God's Holy Day Plan: The Promise of Hope for All Mankind, pp. 24-29.