I understood it like this: (Dates are listed night before day, so the night of Sunday would be The night before Sunday)
The Jews left Egypt the night of the fifteenth of Nissan. (Day one of Pesach): (Shemos 12:6,12)
וְהָיָ֤ה לָכֶם֙ לְמִשְׁמֶ֔רֶת עַ֣ד אַרְבָּעָ֥ה עָשָׂ֛ר י֖וֹם לַחֹ֣דֶשׁ הַזֶּ֑ה וְשָׁחֲט֣וּ אֹת֗וֹ כֹּ֛ל קְהַ֥ל עֲדַֽת־יִשְׂרָאֵ֖ל בֵּ֥ין הָעַרְבָּֽיִם...׃
וְעָבַרְתִּ֣י בְאֶֽרֶץ־מִצְרַיִם֮ בַּלַּ֣יְלָה הַזֶּה֒ וְהִכֵּיתִ֤י כָל־בְּכוֹר֙ בְּאֶ֣רֶץ מִצְרַ֔יִם מֵאָדָ֖ם וְעַד־בְּהֵמָ֑ה וּבְכָל־אֱלֹהֵ֥י מִצְרַ֛יִם אֶֽעֱשֶׂ֥ה שְׁפָטִ֖ים אֲנִ֥י יְהוָֽה׃
You shall keep watch over it until the fourteenth day of this month; and all the assembled congregation of the Israelites shall slaughter it at twilight...
For that night I will go through the land of Egypt and strike down every first-born in the land of Egypt, both man and beast; and I will mete out punishments to all the gods of Egypt, I the LORD.
They slaughtered the Pesach on the fourteenth at twilight, and left that night.
From then, we can assume the Jews were in the desert for three days before Pharaoh decided to chase them, since the original deal was that the Jews would leave for three days before returning, as we see (Shemos 10:23):
דֶּ֚רֶךְ שְׁלֹ֣שֶׁת יָמִ֔ים נֵלֵ֖ךְ בַּמִּדְבָּ֑ר וְזָבַ֙חְנוּ֙ לַֽיהוָ֣ה אֱלֹהֵ֔ינוּ כַּאֲשֶׁ֖ר יֹאמַ֥ר אֵלֵֽינוּ׃
So we must go a distance of three days into the wilderness and sacrifice to the LORD our God as He may command us.”
Moshe never asks to leave for good, only to serve HaShem in the desert for three days. Likewise, Pharaoh never offered to let the Jews go for good (Shemos 12:31):
וַיִּקְרָא֩ לְמֹשֶׁ֨ה וּֽלְאַהֲרֹ֜ן לַ֗יְלָה וַיֹּ֙אמֶר֙ ק֤וּמוּ צְּאוּ֙ מִתּ֣וֹךְ עַמִּ֔י גַּם־אַתֶּ֖ם גַּם־בְּנֵ֣י יִשְׂרָאֵ֑ל וּלְכ֛וּ עִבְד֥וּ אֶת־יְהוָ֖ה כְּדַבֶּרְכֶֽם׃
He summoned Moses and Aaron in the night and said, “Up, depart from among my people, you and the Israelites with you! Go, worship the LORD as you said!
Meaning that Pharaoh only let them go for three days, and after three days he would expect them back!
Assuming the Jews, on foot, with kids and animals, were walking half as fast the Egyptians, who started chasing them the night of the fourth day since they left (the eighteenth of nissan, and the night of the fourth day of pesach), the Egyptians would catch up to them by the night of the seventh day since they left Egypt (the night of the 21, the seventh night of pesach).
The Egyptians then camped near the Jews for the night (shemos 14:20):
וַיָּבֹ֞א בֵּ֣ין ׀ מַחֲנֵ֣ה מִצְרַ֗יִם וּבֵין֙ מַחֲנֵ֣ה יִשְׂרָאֵ֔ל וַיְהִ֤י הֶֽעָנָן֙ וְהַחֹ֔שֶׁךְ וַיָּ֖אֶר אֶת־הַלָּ֑יְלָה וְלֹא־קָרַ֥ב זֶ֛ה אֶל־זֶ֖ה כָּל־הַלָּֽיְלָה׃
and it came between the army of the Egyptians and the army of Israel. Thus there was the cloud with the darkness, and it cast a spell upon the night, so that the one could not come near the other all through the night.
Seemingly the next day, which was The seventh day of pesach The Jews crossed Yam Suf (Shemos 14:21):
וַיֵּ֨ט מֹשֶׁ֣ה אֶת־יָדוֹ֮ עַל־הַיָּם֒ וַיּ֣וֹלֶךְ יְהוָ֣ה ׀ אֶת־הַ֠יָּם בְּר֨וּחַ קָדִ֤ים עַזָּה֙ כָּל־הַלַּ֔יְלָה וַיָּ֥שֶׂם אֶת־הַיָּ֖ם לֶחָרָבָ֑ה וַיִּבָּקְע֖וּ הַמָּֽיִם׃
Then Moses held out his arm over the sea and the LORD drove back the sea with a strong east wind all that night, and turned the sea into dry ground. The waters were split,
So the Jews crossed the Yam Suf on the seventh day of Pesach