Though the Torah declares Passover to be 7 days, the 8th day has an ancient source.

In the times of the Holy Temple, the Sanhedrin actually determined the date of Yom Tov, based on the citing of the moon. Information as to the proper date of Yom Tov often did not reach the Diaspora communities until many days later, and they would celebrate two days of Yom Tov out of doubt.

In the 5th century BCE, when Jewish unity was threatened by the exile from Israel, the patriarch Hillel II set a perpetual calendar and instituted an official "Second Day Yom Tov."

They did this even though they themselves had full awareness of the precise dates of all the holidays. The Talmud (Rosh Hashana 25a) already had pinpointed the length of the lunar month as 29.53059 days (later verified by NASA scientists using satellites, hairline telescopes, laser beams and super-computers).

So why was a second day Yom Tov added for all time? In order to make a distinction, to add to the Jewish awareness that one is living in the Diaspora and does not claim permanent residence in the Holy Land.

And thus the 8th day of Passover.