ליטעראַטור פֿון בית שני אויף דברי הימים ב 7:9

because when the Greeks entered the Temple they defiled all the oil there. When the hand of the Hasmoneans was made strong and they defeat the Greeks, they checked (in the Temple) and only found one jar of oil sealed with the mark of the High Priest which remained undefiled. Though there was only enough in it to light for one day, a miracle occurred through it and they lit the Temple lamps from it for eight days. The following year they decreed these to be eight days of celebration. And what justification did they see for making Channuka eight days? Wasn’t the dedication (channuka) which Moshe did in the wilderness only seven days? As it says “And you shall not leave the entrance of the Tent of Meeting for seven days…” (Vayikra 8:33) And it says “The one who brought his offering on the first day was…” (Bamidbar 7:12) and on the seventh day Ephraim made his offering. So too we find at the dedication which Shlomo made that it only lasted seven days, as it says “…for the inauguration of the altar they made seven days, and the feast seven days.” (Divre HaYamim II 7:9) So what reason did they see to make this dedication eight days? In the days of the kingdom of Greece the Hasmoneans entered the Temple, built the altar, plastered it with plaster and for seven days they were preparing the vessels for service. The re-dedication done by the Hasmoneans it to be marked for all generations. And why is it a practice for all generations? They fixed it when they came out from a narrow place into broad spaces, and they said praises and thanksgiving, lighting lamps in purity. Since the Greeks had gone into the Temple and defiled all the vessels, there was nothing with which to light. When the Hasmoneans were victorious, they brought seven skewers of iron, covered them with tin and began to light.

Why did they see fit to make Chanukkah eight days? Was it not that the dedication [chanukkah] that Moses did in the wilderness was only seven days, as it says, You shall not go outside the entrance of the Tent of Meeting for seven days, [until the day that your period of ordination is completed. For your ordination will require seven days.] (Leviticus 8:33), and it says, The one who presented his offering on the first day... (Numbers 7:12), and on the seventh, Efraim offered (see Numbers 7:48). And so too we find in the dedication [chanukkah] that Solmon did, that he only did seven days, as it says, On the eighth day they held a solemn gathering; they observed the dedication of the altar seven days, and the Feast seven days. (II Chronicles 7:9). So why did they see fit to do this dedication [chanukkah] for eight days? Because in the days of the rule of Greece, the Hasmonean clan entered the sanctuary and built the altar and plastered it with plaster and fixed the sacred implements there, and they were occupied by it for eight days.