The Torah commands us to celebrate the first and 7th day of Passover as a special Yom Tov.
The first day marks our exodus out of Egypt. But this exodus was not complete, since Bnei Yisrael were still in danger of being destroyed by Paraoh’s army.
The final exodus was actually on the 7th day, the day HaShem miraculously parted the sea. No longer did the Jews fear the Egyptians.