God told Moses that He was going to go throughout Egypt at midnight and kill all of the eldest of the Egyptians and the firstborn of their animals (Exodus 11:4). Evidently as Moses was leaving Pharaoh that last time, this message came forth from the Lord. As Moses was departing then he gave Pharaoh the message that is recorded in verses 4-8. It was a threat, a promise and a prophecy. And, Moses delivered it furiously (Exodus 11:8b).
God encouraged Moses in several ways on this day.
1. God told him that the Egyptians would not only cease detaining the Jews, they would actually "chase" them out of Egypt.
2. God informed Moses that they would go out rich after looting the Egyptian people... well, spoiling them at least. It was promised that the Egyptians would willingly surrender their wealth to the Hebrews.
3. God let him know that these Jews who had been despised by the Egyptians for so long would suddenly be admired by their former opressors.
4. God promised absolute safety for the Jews in their escape.
5. God gave Moses assurance that (even though the years of captivity were over) He was going to deal with Pharaoh and his wicked people.
This reminds me of the midnight hour visitation mentioned in Matthew 25:6, "At midnight there was a cry made, 'Behold, the bridegroom cometh; go you out to meet him.” The wise virgins were ready, and the foolish virgins suffered loss (Matthew 25:10 & 12). Truly the midnight appointment means two very different things to two very different groups of people. God's presence was fatal to the rebels and delightful to the redeemed.