Here is a compilation of answers to the famous question from the Bais Yosef. Why do we keep 8 days of חנוכה if the נס was 7 days since there was enough oil to last for one day?
1. The יונים ransacked the בהמ"ק many days in search of oil to make it טמא. Despite their strength and numbers, they overlooked one flask. A few weak, battle-weary Jews found it immediately. We celebrate the first day to commemorate the miraculous victory over the Greeks.
2. It would take 8 days to get oil so the Jews divided the oil which contained enough for one night into 8 equal parts. They figured they would light one-eighth of the oil each of the next eight nights. Miraculously, on each of the eight nights, the oil lasted for the entire night.
3. After they filled the menorah with oil on the first night, the jug remained full on all eight days.
4. The oil in the cups never burned, the cup remained full the next morning.
5. ברית מילה was not allowed and after the war they were allowed to do it openly so for the fact that after a baby boy is born we wait 8 days, we have 8 days of חנוכה.
6. They needed thicker wicks but they used wicks 1/8th of the normal thickness so it would last for more days. It looked like it was very thick from far and brightened the whole ירושלים. So we light because the first day it was also a נס.
7. They were not allowed to make a copy of the כלים in the בית המקדש so that’s why we make an 8 branch and not the 7 branch.
8. The משכן was completed on כ"ה כסלו but it was only dedicated in ניסן. To make it up to חדש כסלו, ה' caused נס חנוכה to occur in כסלו. The dedication of the משכן and the beginning of the עבודה lasted 8 days so we celebrate חנוכה for 8 days. The meaning of חנוכה is dedication- this eludes to this aspect of the celebration.
9. The first day commemorates the dedication of the second בית המקדש –when it was built.
10. It was a נס that they found a פך שמן טהור with the seal of the כהן גדול.
11. The oil was enough for the first night but the מנורה must be lit by day too.
12. They were forbidden to extinguish the fire on the morning of the first day in order to conserve oil yet the oil continued burning through the eighth evening-(זית רענן)
13. The eighth day was celebrated out of doubt just like all ימים טובים outside of ארץ ישראל. (ברקי יוסף, תולדות יעקב ויוסף).
14. They planned on using multiple wicks dipped in oil but the original wicks lasted all day.
15. The candles didn’t burn out despite the wind outside. They lit the מנורה outdoors because the בהמ"ק was full of ע"ז.
16. That year כסלו was short but nowadays its long (29 days versus 30 days). We still do the יו"ט from כ"ה כסלו through ב' טבת even though it is now 8 days rather than only 7.
17. The מצוה of lighting the מנורה comes right after the מצוה of keeping 8 days of סוכות (7 days+שמעיני עצרת) The חכמים took it as a hint that חנוכה should be 8 days too.
18. The נס is not in the quantity of the oil but quality. That is burned 8 days slower than usual which also occurred on the first day.(ר' יוסף אנגעל)
19. The oil was enough for lighting מנורה but not for relighting the western light in the morning as it was each day.(חמדת שלמה )
20. They lit the מנורה before dark on the evening of כד' כסלו so the נס also applied to the end of the first day.
21. We know that the oil was supposed to burn for one day and the miracle was that it burned for eight days. But, even the oil burning for one day isn’t “supposed” to happen. Everything that happens, even the things that seem like they happen naturally, daily are miracles.טבע is really נס.
22. The Zohar says: A bracha can only catch onto something that exists-like when we say Birchas Hamazon we have a piece of bread on the table so that the bracha could catch onto it. So on the first night, the oil burned the full amount of time but obviously, not all the oil was used because there had to be oil for the next day for the bracha to catch onto So it’s a miracle that it burned the full amount of time but didn’t use all the oil.
23. The Jews were impure from the battle. They had to use an earthenware menorah temporarily. Because it’s porous, one jar wasn’t enough for the whole day.
24. That year, כה' כסלו was on Shabbos so they needed extra oil because they had to light the מנורה before Shabbos.
25. They filled the מנורה, and the next day, it was still full.
26. When they filled the מנורה, the oil stuck to the jug and so not enough was put in yet it still burned the whole night.
27. Other Jews lit torches which did not burn out until the end of the 8 days.
28. They poured a small amount of oil into the מנורה and it filled it all up.
29. The מנורה needed 3 ½ לוגין of oil. The flask only had 3לוגין .
30. So that the last day of חנוכה will be the same day that the miracle happened.
31. Some say that the miracle was 8 days- not 7. (Sefer Pischei Olam)
32. The flame was lit but no oil was being burnt.
33. They used aמנורה that was broken and could not contain a full day’s amount of oil but still, it miraculously burned for a whole day.
34. They were such צדיקים that when they lit it with such excitement, it should have toppled over but it didn’t. Every day was a נס.