The Festival of Lights
The eight-day festival of Hanukkah falls on the 25th of Kislev.
It was on this day that the small Jewish army defeated the mighty Syrian-Greek army of Antiochus.
The victory enabled the Jews to once again enter the Holy Temple in Jerusalem in order to rededicate the Temple, the holy vessels and themselves to the service of God.
In order to light the Menorah (golden candelabra), the priests needed oil that was pure and undefiled. In the whole Temple only one small jar of pure oil could be found. This oil was only sufficient to last for one day. Miraculously. it lasted for eight days.
Hence, Hanukkah is eight days long to commemorate this event.