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.
 There are additional examples as well. Because of the Hasmonean revolt, the wicked Antiochus went up to Jerusalem to destroy the city and annihilate the Jews. Upon hearing troubling reports of rebellion in the eastern part of his kingdom, however, he was forced to end the siege on Jerusalem on the 22nd of Shevat (167 BCE). He was eventually killed in the rebellions. The 3rd of Kislev: on this day, the Hasmoneans removed the emblems of the Greek troops from the Holy Temple. On the 24th of Av, they reinstated Torah law as the law by which the Jews adjudicate themselves, instead of Greek law. The 23rd of Iyar marked the day on which Shimon son of Matityahu the Hasmonean conquered the Fortress of Chakra, in which there remained a Greek garrison even after Jerusalem was liberated. On the 27th of Iyar the Hasmoneans (apparently during the rule of Yonatan son of Matityahu) abolished the signs of idolatry that hung upon the entrances of the houses and stores. The 15th and 16th of Sivan: this is when the Hasmoneans conquered Beit Sha’an and drove out the heathens who oppressed the Jews. The Sages also established holidays when the evil kings who persecuted them died: King Yannai on the 2nd of Shevat and King Herod on the 7th of Kislev. Many other dates are mentioned in Megillat Ta’anit.