There is a classic question of the Beit Yosef that we mention every year. Chanukah really should be seven days and not eight. The cruse of oil was supposed to burn for one day. Therefore, the first day was not a miracle. The extra seven days that it lasted was the real miracle. So why eight days and not seven?
There are numerous answers to this question. Some say the first day is a celebration of the military victory and the other seven days was the miracle of the oil. Others say the oil was divided into eight equal parts so that the miracle was seen on the first day.
There is an opinion that the pitcher of oil remained full immediately after pouring it out on the first day.
My favorite answer was given by Rabbi Meir Kahane זצ״ל. He said that the reason for the celebration of the first day was the miracle that Jews "got involved" altogether. And the other seven was the miracle of the oil. Chanukah Sameach