The Bais Yosef famously asks1 why we celebrate Chanukah for 8 days when the actual miracle occurred for 7 days. There are many answers to this question but here are “8” interesting ones…

1. The pure oil was made in a location that was 4 days’ travel from the Beis
HaMikdash. It took 4 days to get there and another 4 days to return. Since they could not travel on Shabbos, the oil only arrived at the Beis HaMikdash on the 9th day. The first day was not a miracle, but the other 8 were miraculous.

2. When they filled up the menorah, the oil jug miraculously remained full.

3. One of the things that the Greeks wanted to stop was the mitzvah of bris milah, so for that reason it was decided to celebrate Chanukah for 8 days.

4. In the year that the Chanukah miracle took place, the 25th day of Kislev was on Shabbos, which meant they needed to light the menorah on Friday
and they would have needed more than the regular amount of oil for the first day. The lone jug they found had exactly the amount that was needed for 24 hours, but a miracle happened and the oil burned until the end of Shabbos, so even the first day there was a miracle.

5. We celebrate the first day to remember the joy the Chashmonayim had when entering the Beis HaMikdash and that were not discouraged or downcast by all the damage that was done. They were so happy to be able to return and rebuild the Beis HaMikdash that Hashem made a miracle that the jug of pure oil burned for eight days.

6. The number eight represents going above nature. We celebrate for 8 days to emphasize the tremendous miracle that occurred when the tiny Chashmonayim army defeated the massive force of the Greeks.

7. The first day is a celebration of the miracle of finding a pure jug of oil.

8. Since the Chashmonayim knew that they would not have oil for another 8 days, they decided to divide the oil that they had into equal parts so the menorah could at least burn for 3 hours each day. The little bit of oil they put in miraculously lasted 24 hours every day for eight days.

1 Orach Chaim 670.