One could ask why they established an eight day celebration at all. After all, since there was enough oil in the sealed jar to light for one night then the miracle actually was for seven nights.

1. One could answer that they divided the oil they found into eight parts and placed one portion in the menorah each night, which nonetheless burned until morning. Thus a miracle occurred every night.

2. Further you could say that after they filled the menorah on the first night the jar remained full of oil as it was in the beginning, and thus the miracle was recognizable even on the first night.

3. Alternatively, they poured all the oil into the menorah on the first night and though the candles burned all night they found them still full of oil in the morning, and so it happened each night.

(48) They made also new holy vessels, and into the temple they brought the candlestick, and the altar of burnt offerings, and of incense, and the table.

(49) And upon the altar they burned incense, and the lamps that were upon the candlestick they lighted, that they might give light in the temple.

(50) Furthermore they set the loaves upon the table, and spread out the veils, and finished all the works which they had begun to make.

(51) Now on the five and twentieth day of the ninth month, which is called the month Kislev, in the hundred forty and eighth year, they rose up betimes in the morning, And offered sacrifice according to the law upon the new altar of burnt offerings, which they had made.

(52) Look, at what time and what day the heathen had profaned it, even in that was it dedicated with songs, and citherns, and harps, and cymbals.

(53) Then all the people fell upon their faces, worshipping and praising the God of heaven, who had given them good success.

(54) And so they kept the dedication of the altar eight days and offered burnt offerings with gladness, and sacrificed the sacrifice of deliverance and praise.

What is Hanukkah, and why are lights kindled on Hanukkah? The Sages taught in Megillat Ta’anit: On the twenty-fifth of Kislev, the days of Hanukkah are eight. One may not eulogize on them and one may not fast on them. What is the reason? When the Greeks entered the Sanctuary they defiled all the oils that were in the Sanctuary. And when the Hasmonean monarchy overcame them and emerged victorious over them, they searched and found only one cruse of oil that was placed with the seal of the High Priest. And there was sufficient oil there to light the candelabrum for only one day. A miracle occurred and they lit the candelabrum from it eight days. The next year the Sages instituted those days and made them holidays with recitation of hallel and special thanksgiving in prayer and blessings.