The Torah tells us to celebrate Pesach for 7 days, which they do in Israel. We celebrate in the Diaspora for 8 days because the Rabbis outside Israel were uncertain as to which day was the last day of Pesach. Messengers proclaiming Rosh Chodesh, the first of the month from which all festivals are set, travelled 11 days, so Jews outside this radius kept an extra day to be certain that the festival was celebrated correctly.
The number 8 is an important number in Judaism. The menorah stayed alight for 8 days in the Temple on Chanukah. Brit Milah is carried out on the 8th day after the birth of a baby boy. Some rabbis say this lends an extra spiritual dimension to the natural world. 8th day Pesach is traditionally associated with the coming of Moshiach. The Haftarah makes many references to the era of redemption, as in Isaiah’s prophecy “the wolf will dwell with the lamb; the leopard will lie down with a young goat.” 250 years ago the Ba’al Shem Tov started a custom to connect redemption with the 8th day Pesach. He would hold a seudah for Moshiach, the festive feast. The main ingredient of this feast in the time of the Ba’al Shem Tov, was matzo because its tasteless flatness symbolises humility.
Why do we not say shehecheyanu when we light the candles on the last two day of Pesach? I found an inspiring explanation on the Internet. Eli Block says: the first days of Pesach commemorate the Exodus from Egypt and the freedom we received, both physical and spiritual. They celebrate the past. 7th and 8th days Pesach are looking to the future – the final redemption and the coming of Moshiach. However this promise is yet to be fulfilled. The world is still at war; poverty and malice still exist, therefore how can we thank Hashem when Utopia has not been reached? So the Rebbes concluded that to say shehecheyanu for the last two days would be premature.
8th day Pesach looks forward to a new redemption and exodus, when the Jewish people will be gathered, not from Egypt but from all the lands of the Diaspora, to Eretz Yisrael. Isaiah 11:11 foretells this: “In that day the Lord will reach out His hand a second time to reclaim the remnant of His people…”