According to the Mechilta commentary, the chronology of events that transpired between the first and seventh days of Pesach was as follows:

During the entire period when Moses negotiated with Pharaoh, he asked only that Israel be permitted to travel for three days to offer sacrifices to G‑d. His purpose was to establish Israel as men set free by G‑d rather than slaves set free by Pharaoh. When they left, Pharaoh knew that they had no intention of returning. Their G‑d, who had performed incredible miracles in Egypt, would feed and sustain them in the wilderness. Nonetheless, Pharaoh thought: "They asked me for no more than three days, so it is as if I still have a hold over them." He therefore dispatched couriers to accompany them and report back to him as to their actions. Israel did not prevent these couriers from going with them.

On the morning of the fifteenth of Nisan, all of G‑d's hosts departed from Ramses and on that same day they arrived in Sukkos. There the Holy One, blessed is He, encompassed them with seven clouds of glory. As we have already noted, that day was a Thursday.
The next day, the sixteenth of Nisan, they traveled from Sukkot and encamped at Etam on the edge of the wilderness. On the seventeenth of Nisan, which was Shabbat, they remained encamped.

On Sunday the eighteenth of Nisan, the people of Israel began to prepare their belongings and animals for departure. Pharaoh's couriers said to them:

"Your period of freedom has ended, it is time for you to return to Egypt, for you said that you would be going on a three-day journey."

Israel replied: "It was not by Pharaoh's permission that we left Egypt. It was G‑d's exalted Hand that brought us out."

The couriers countered: "Whether you like it or not, in the end you must obey the royal command."

Israel rose up against them and struck them, killing some and injuring others. Those who remained went back to report to Pharaoh.

When the couriers left at midday of the eighteenth of Nisan, Moses said to Israel: "Go back toward Egypt so that Pharaoh shall not claim that you are fleeing. Let him catch up with you near his land and if he has the power to stop you, let him come and stop you." Moses sounded the shofar and the people returned to Pi haChirot, a day and a half's journey from Egypt.

When the blast of the shofar was heard, those with little faith began to tear out their hair and rend their clothes, for they thought that Moses was returning them to Egypt. They were calmed only when Moses told them: "G‑d Himself has told me that you are free men. Our apparent retreat is only to entice the Egyptians and mislead them."

The couriers traveled for a day-and-a-half and at the end of Monday, the nineteenth of Nisan, they came to Pharaoh and informed him that the people had fled.

On Tuesday, the twentieth of Nisan, Pharaoh assembled his chariots and, gathering his nation to accompany him, set out in pursuit of the Children of Israel, catching up to them as they encamped on the banks of the sea.

On Wednesday, the seventh night of Pesach, the beginning of the twenty?first of Nisan, Israel entered the sea and in the morning they came out and saw what G‑d's exalted Hand had done to the Egyptians. It was then that Moses and the Children of Israel sang their song of praise.

It was on this very date, eighty-one years before Israel stood on the banks of the Red Sea, that Moses was cast into the river as a result of Pharaoh's edict. The Talmud (Sotah 12b) writes: R. Chanina bar Pappa taught: That day, when the infant Moses was cast into the river, was the twenty-first of Nisan. The ministering angels said to G‑d: "Master of the World, he who is destined to sing the Song of the Sea on this day, shall he be punished on this day?"

On this same date, one year before the Exodus, Moses left Midyan at G‑d's command to take Israel out, for on the fifteenth of Nisan, G‑d had appeared to him from within the bush and urged him for seven days - the period of Pesach - to accept this mission.