The Shehechiyanu blessing is said to acknowledge the miracle of the present moment. Traditionally, it is recited when we do something for the first time that year — such as lighting Chanuka candles, hearing the shofar, or shaking the lulav and etrog — as well as at the start of most Jewish holidays. The blessing honors and expresses the wonder of having arrived. “Blessed are You, L-rd our G‑d, King of the Universe, who has granted us life, sustained us and enabled us to reach this occasion.”
Tonight, we’ll be celebrating the last days of Passover, known as “Shvi’i Shel Pesach” and “Acharon Shel Pesach.” When we light the Holiday candles, however, we will not be reciting this blessing, even though it is the start of the “last days”.
The reason for this is that the last days of this holiday commemorate the drowning of the Egyptians in the Red Sea. We are celebrating the destruction of evil - something which is not yet complete. There is still evil in this world, which we must work hard to eradicate. This process will be complete with the coming of Moshiach.
Therefore, during the afternoon of the last day of Passover, which falls out on Shabbat this year, we sit down for a Moshiach Seudah - the feast celebrating the redemption. We drink 4 cups of wine and eat Matzah to start the meal that we will complete with Moshiach, when we leave this final exile, may it be now!
Tonight, light Holiday candles before 7:24 PM. Tomorrow evening, light Shabbat/Holiday candles before 7:25 PM (from a pre-existing flame). For the blessings and more, check out http://Passover.org