When they sought to light the Temple's menorah (the seven branched candelabrum), they found only a single cruse of olive oil that had escaped contamination by the Greeks; miraculously, the one-day supply burned for eight days, until new oil could be prepared under conditions of ritual purity.
The joyous festival of Hanukkah begins on 25 Kislev of the Jewish calendar. It celebrates two miracles --a great Jewish military victory and a miraculous supply of oil for the Temple.