The Mysterious Date of Chanuka, by Rav Yitzchak Etshalom

Why does Chanuka begin on the 25th of Kislev? 

Every other day that is marked in our calendar for celebration has a clear source as to why that date was chosen. In the case of festivals marked in the Torah - that is the source. In the case of Purim, the text records that the Jews of the Persian Empire celebrated on the 14th (and, in Shushan, on the 15th) of Adar and declared that those days will be kept for generations. There is, however, no clear source as to why Hanukkah begins on the 25th. Suggestions, such as that that was the day the battle for Yerushalayim was won or that was the day that the Mikdash was finally under Jewish control don't fit with what we know about the history of the event. The Mikdash had to be cleansed, the defiled altar destroyed and the new one built. Furthermore, there is evidence in Megilat Ta'anit that the Hashmonaim had control of the Mikdash already in the month of Marcheshvan - over 4 weeks earlier. Why did they wait to renew the Avoda? This mystery and a number of others are addressed in this shiur.