There is a famous question asked by the Beis Yosef (R' Yosef Karo): Why do we celebrate eight days of Chanukah if the miracle lasted only seven? The jug of oil contained enough to be lit for one day, so the first day was not a miracle.
This is a famous question asked by the Bait Yosef (OC 670), which has gained so much popularity since he's asked it, seemingly because of it's simplistic ingenuity, that it's attracted hundreds of answers from most everyone who has ever had anything to say about Chanuka.
The Bait Yosef himself gives three possible answers: (quoted from here)
Those who were preparing the Menorah for lighting knew that it would take eight days until new oil could be obtained. They therefore divided the flask into eight parts, so that at least the Menorah would be lit every day, albeit not for the entire day. A miracle occurred and the small amount of oil that was placed in the Menorah each day lasted an entire day. Hence, there was a miracle on the first day as well.
On the first night, the contents of the flask were emptied into the Menorah. This would enable the Menorah to be lit for one entire day. However, after filling the Menorah, it was discovered that the flask miraculously was still full. This miracle repeatedly occurred for each of the days. Hence, there was a miracle on each of the eight days.
- On the first night, the entire contents of the flask were emptied into the Menorah. This would enable the Menorah to be lit for an entire day. When the Menorah was checked on in the morning, it was discovered that none of the oil burned up, and the Menorah was still full, although the flame was lit. This miracle occurred for each of the days. Hence, the first day when the oil did not burn up was miraculous as well.
(See the link for more answers.)
However, if you were to ask me, I would give a much more simplistic answer. I believe it was a miracle in the first place that they found any oil at all with which to light the menora, regardless of how long it burned. If the oil had burned only one day, we would celebrate at least for one day by lighting the menora in commemoration. This seems to me enough to justify our saying "She'asa Nisim Lavotenu". Now that it miraculously burned for eight days, we celebrate by lighting for eight days.
Here is something I wrote about this last year:
One of the most famous questions asked about חנוכה is known as “The Beis Yosef’s Question”. The גמרא explains the reason for the celebration of חנוכה is because the Jews found only one flask of oil containing enough oil to light the מנורה for just one day. A miracle occurred and the oil lasted for eight days. If so, the בית יוסף asked, the miracle was only for seven days, so why do we have eight days of חנוכה? Countless answers have been given to this question. In fact, a ספר titled נר למאה collects 100 explanations. Here are some of the more interesting answers. [Although this question is known throughout the Torah world as קושיית הבית יוסף, it is in fact already mentioned by ראשונים such as תוס' הראש and the מאירי].
When the Jews returned to the בית המקדש it had been completely defiled by the Greeks and was littered with their idols. The Jews did not want to wait until they had cleared the בית המקדש, so they lit the מנורה outside. The amount of oil required to keep a flame burning outside, where it is exposed to the wind, is greater than the amount needed inside. When the oil for the Menorah lighting had originally been measured and placed into special bottles, it was done so under the assumption that the candles would be lit inside the Temple, where its flames would not be subject to the wind. Thus, even the first night was a miracle, because the oil should not have been sufficient even for that night! (In על הנסים we say "והדליקו נרות בחצרות קדשך, וקבעו שמונת ימי חנוכה אלו", “And they kindled lights in Your holy courtyards, and instituted these eight days of Chanuka.” Since they lit the candles in the courtyard and not the בית המקדש itself, they thus established eight days of חנוכה and not seven. However, the Lubavitcher Rebbe questions this explanation based on several difficulties, and concludes that this does not refer to the מנורה at all.) (דרשות חתם סופר – חנוכה תקצ"ב. ראה לקו"ש חכ"ה עמ' 235)
The Greeks forbade the Jews from performing the three מצות of ראש חודש, שבת and ברית מילה. (The חיד"א writes that this explains the origin for the name חשמונאי – an acronym for "חודש, שבת מילה – ונס נרות אשריכם ישראל"). On חנוכה when we celebrate our victory over the Greeks, we hint to all three things. During each חנוכה there is a שבת and a ראש חודש, and so they established eight days to commemorate ברית מילה. (בעל העתים, מובא בבשמים ראש על סידור אוצר תפילות)
When the destitute widow of the prophet Ovadiah called out for assistance from Elisha, Elisha asked her, "What do you have in the house?" The widow replied that all she had was a small jug of oil. Elisha then instructed her to borrow empty vessels and to pour into them from the jug. The oil miraculously poured until there were no more vessels available for use. The זהר asks: If Hashem was willing to perform a miracle, what was the difference which possessions she had at home? From here we learn that rather than performing miracles to create a new existence, Hashem sends a ברכה to increase something that already exists. This answers the Beis Yosef's question. There had to have been some oil left over from the first night in order for the miracle to take effect on the subsequent nights. The fact that all the oil did not burn out on the first night was a miracle. (ט"ז או"ח סימן תרע סק"א)
The גמרא tells the following story: Once the daughter of ר' חנינא בן דוסא mistakenly poured vinegar instead of oil into the שבת candles. ר' חנינא famously told his daughter not to worry, as “The same G-d that told oil to burn, can tell vinegar to burn.” Indeed, that week the שבת candles burned until שבת was over. Ultimately, the burning of oil is no less miraculous than the burning of vinegar. The only difference between the two is how frequently they occur. In order to remind us that Hashem dictates nature just as he causes miracles, the חכמים established eight days of חנוכה, the last seven to commemorate the miracle of the מנורה, and the first to remind us that even the normal burning of oil is only in obedience to Hashem’s wish. (בשם ר' משה פיינשטיין)
It is forbidden to replicate the מנורה in the בית המקדש for one’s personal use. The חכמים were concerned that if they would make חנוכה seven days long, the מנורה with seven branches would resemble that of the בית המקדש, and therefore established the festival for eight days. (בשו"ת שואל ומשיב מהדו"ק ח"ג סימן עא)
Since the Greeks defiled the מנורה, the Jews had to construct a new one. When one puts liquids into a כלי some of the liquid becomes absorbed into the surface of the כלי. However an old כלי which has already contained liquids previously does not absorb any more, for it is already saturated by what it had absorbed (see ט"ז יו"ד סימן צג סק"ב). Although the flask they found contained enough oil to last one day in the regular מנורה, since they were using a new one some oil went into the surface and they required a miracle to last even the whole first day. [Based on this we could explain why the מצוה is to light the candles specifically for half an hour. The מדרש שוחר טוב states that during the months from תשרי to טבת the night is 15 hours long. The פרי מגדים writes that the amount of liquid that becomes absorbed into the walls of a כלי is one thirtieth of the total volume of the כלי. If so, there was half an hour worth of oil missing on the first day, and we commemorate this by lighting מנורה for half an hour each night]. (אור התורה מהאדמו"ר מאוסטרובצא זצ"ל – חנוכה אות ח)
The story of חנוכה occurred in the year 3622 from creation (139 BCE). If one would calculate the מולד for כסלו that year, it works out that כ"ה כסלו was a שבת. Since they had to light the מנורה before שבת started, they required a little more oil than a regular night. Thus, even on the first night a miracle occurred. (עצי זית)
The Lubavitcher Rebbe, after bringing several answers and refuting them based on Halachic objections, answers as follows: The ultimate miracle is the merging of the divine and the natural; that physicality itself is elevated to function on a level higher than the mundane. The miracle of oil was in that the physical oil burned but simultaneously it did not become consumed (similar to the burning bush Moshe witnessed). A similar miracle occurred constantly in the קודש הקדשים were the ארון did not take any physical space, yet at the same time would only be considered fit for use if it satisfied the correct measurements required by the תורה. Similarly, during all eight days of חנוכה the oil burned naturally but did not become consumed; a miracle on every day. (לקו"ש חלק טו עמ' 183. וכבר הקשו רבים ע"ז א"כ מה הי' הנס ביום הח', ואכמ"ל)
The answer as I remember is this...since it takes eight days for new shemen to be manufactured for the ner tamid, the scholars at the time said "let's use 1/8th of the oil each day so that the ner will burn for at least three hours a day." However, the shemen burned for 24 hrs each day. Therefore the miracle was NOT that one day's supply of shemen lasted for eight (a 7 day miracle), but that 3 hours worth of shemen lasted 24 hours each day for 8 consectuive days...therefore providing for an 8 day miracle.
I'm not sure what the general policy is around here regarding quoting books of the Apocrypha, but the Book of Maccabees gives a totally different explanation, nothing to do with oil: the eight days was meant to parallel Sukkos, since the Jews were unable to celebrate Sukkos of that year due to the Greeks having taken over the Beis Hamikdash. (Macabees II 10:5-6)
The Beis HaLevi writes that the miracle was that the sages of the time, in order to ensure that the oil would last for the whole 8 days, only poured 1/8th of the oil into the Menorah each night, and they made the wicks 1/8th of their usual size, so it would burn slower, albeit with a smaller flame. Hashem made a miracle that the flame burned as brightly and beautifully as if the wicks were normal size. Therefore, the miracle was on all 8 days. It is for this reason, the Beis HaLevi explains, that we have a special concept of "hiddur mitzvah" -beautification - with the Chanukah Menorah, beyond the norm for other mitzvos - because the miracle that Hashem did for us was to beautify the Menorah.
Bais Yosef asks if the ness of the oil was for only 7 days (the 1rst day they had the pach shemen) why do we celebrate 8? one of the Bais Yosef's answer is that they divided the oil into 8 parts. The problem is how were they allowed to light only 1/8 of the pach ?(because it wouldnt last the whole night)
I am looking for a source if it would be possible to put for example: tameh oil (which is mutar if no tahor oil is available) in the same receptacle (attached to the ground)with the tahor oil without making the tahor oil tameh? (example: cubes of solidified oil, or separate compartments...with the wick going to both parts)
This would solve the problem of having enough oil to light: 1/8 tahor + 7/8 tameh is a shir that lechatchila they could light. THEN the ness could happen that only the 1/8th part would burn.