http://revach.net/article.php?id=1494

Section: Moadim   Category: Chanukah
Another Reader's Answer to the Bais Yosef's Question: The Whole Point of a a Nes
A reader calling himself "Nisim" sent this beautiful answer to the Bais Yosef's question explaining why we celebrate the Nes for 8 days.  Really, if there was one Pach Shemen we only needed to rely on a Nes for Seven days.

A Nes is not about watching the supernatural happen.  It is about recognizing Hashem.  The word Nes actually means tall pole as we see in Bamidbar where Hashem tells Moshe to put a copper snake on a Nes.  When Hashem's hand in the world is clear we call it a Nes because it usually hides and doesn't stand out. 

Therefore one can make the argument that the eighth day was the greatest "Nes".  The first seven days a great miracle happened as the oil lit and didn't burn out.  While there was great joy, the miracle was not complete.  There were still more days to worry about.   Once the seven days past and the new oil was on its way back to Yerushalayim for tomorrow they no longer needed to "wait on shpilkes".   The they were able to completely focus on the great miracle that happened and truly thank Hashem for the victory in war and the restoration of the Bais HaMikdash. 

This recognition happened on the eighth day.  Not that I was there but I can imagine the singing and dancing and hakoras hatov and appreciation of the Ribono Shel Olam that went on all night long as they joyfully celebrated the Nes of Chanukah.  The dveykus probably lasted until the morning and culminated with a gala seudah in honor of the people who brought back the oil and in honor of Hashem.

For more answers and reader discussion on this question you can go to the Help Wanted article "7 Days Miracle or 8"