You can find ALL of the laws of Hanukkah – in “Peninei Halakha” Online
During the Second Temple era, “The Greeks entered the sanctuary and defiled all the oil that was there. When the Hasmonean dynasty gained power and defeated them, they searched for pure oil and found only one flask that was marked with the seal of the High Priest, but it had enough oil only to light the Menorah one day. A miracle occurred and they lit the Menorah for eight days with this oil. A year later, the Sages established these days as a holiday, making them a time for praise and thanksgiving,” and one may not fast or deliver eulogies on these days (Shabbat 21b, Megillat Ta’anit 9:2).
The Sages established many more holidays for the Jews during the Second Temple era, to thank God and rejoice over the salvations He performed for Israel. They are all mentioned in an ancient scroll called Megillat Ta’anit. Many of these holidays commemorate the victories of the Hasmoneans, like “Nicanor Day” – the 13th of Adar – on which the Hasmoneans defeated a large Greek army and killed their commander, Nicanor. On the 14th of Sivan, they conquered Caesarea. On the 22nd of Elul, they killed the apostates who refused to repent. On the 23rd of Mar-Cheshvan, the Hasmoneans destroyed the brothel that the Greeks had built near the Holy Temple. On the 25th of the same month, they conquered Samaria and began settling it1.
However, the halachic authorities determined that the holidays enumerated in Megillat Ta’anit were annulled after the destruction of the Second Temple (Shulchan Aruch, Orach Chayim 573:1). After all, once the Temple was destroyed, all the good things that happened on these days faded away and there was no longer any reason to celebrate them. It is even permissible to fast and deliver eulogies on these days. Chanukah is the only holiday that retained its special status and remains in effect throughout the generations. The Sages explain that this is because of the special miracle that took place with the oil-flask and the mitzvah of lighting the candles that the Rabbis enacted to publicize the miracle. And since we already keep the mitzvah of lighting the Chanukah candles, we also preserve the other aspects of the holiday: we insert Al HaNissim into our prayers, recite Hallel to praise and thank God for saving His nation, and refrain from fasting and delivering eulogies throughout the holiday (see Rosh HaShanah 18b, with Rashi and Ritva).
In order to better understand the significance of Chanukah and the miracle of the oil-flask – the only remnants of all the holidays that existed during the Second Temple era – we must elaborate a bit on the events that occurred in those days and explain their meaning.
