You shall keep the Festival of Unleavened Bread. For seven days.
The Ohr Hachaim explains the juxtaposition of this possuk to the previous Parsha. The Torah has just finished the distasteful story of the Golden Calf, how Klal Yisroel slipped from their lofty level of Kabbalas Hatorah and worshipped idols. When they did Teshuva and Hashem accepted them, they were commanded about the Yamim Tovim.
The Zohar writes that Chametz is a strange god inside a person. When Klal Yisroel were cleansed from their impurities, they symbolize that by keeping away from leavened bread. After having worshipped the Golden Calf, they needed an extra warning to keep away from strange gods. Even though they were already commanded at Har Sinai, it seems that they still had a small attachment to idol worship, that could flare up in the future. They needed to remind themselves yearly to keep far away from this problem so that it does not reoccur.
The Ohr Hachaim then explains why all of the Yamim Tovim were repeated in this Parsha. The Yamim Tovim were all instituted as a reminder of the Exodus from Mitzrayim, as we say multiple times in the davening of Yom Tov and Kiddush. Klal Yisroel said יךָ יִּשְׂרָאֵל אֲשֶר הֶעֱלוּךָ וֹקאֵלֶה אֱל מֵאֶרֶץ מִּצְרָיִּם - This is your god, Israel, who brought you up from the land of Egypt. They renounced the original exodus through Moshe from Hashem and did not learn the lesson of the Yamim Tovim.
Part of Teshuva is reversing the mistake that was made. If they did not remember that it was Hashem who took them out of Mitzrayim, they needed to be reminded of that lesson through the commandment of the Yamim Tovim.
They also had to learn the Parsha of Shabbos again, as we see in the possuk later שֵשֶת יָמִּים תַעֲבֹד וּבַיּוֹם הַשְבִּיעִּי תִּשְבֹת - Six days you shall work and on the seventh day you shall cease. Klal Yisroel worshipped idols, which is a terrible aveira, equal in magnitude to the entire Torah. The only solution to this terrible problem, the only repair for this ruin, is to be careful to keep Shabbos, which is also equal in magnitude to the entire Torah.
