Hanukkah seems to begin on a different date every year, but it actually occurs on the same date every year. It is just that the date is not on the secular calendar, which is also called the Gregorian calendar, that is used around the world in modern times. The date is on the Jewish calendar. There are differences between the Jewish calendar and the Gregorian calendar. Another reason why Hanukkah seems to be on a different day every year is that it is not a one day holiday. It actually lasts slightly more than a week.
Hanukkah is a Jewish holiday. The Jewish calendar is a lunar calendar which means it is based on the moon’s rotation around the earth. The Gregorian calendar is a solar calendar and is based on the earth’s rotation around the sun. Because one calendar is based on the moon and the other is based on the sun, two different cycles are in use. That is why the dates are different every year on the Gregorian calendar and Hanukkah does not begin on the same day on the Gregorian calendar every year. However, Hanukkah does begin every year of the Jewish calendar on the same day. Hanukkah begins on the 25th day of the Jewish month of Kislev. The 25th of Kislev could fall anywhere from late November to late December on the Gregorian calendar. For example, in 2010, Hanukkah began on December 2. In 2011, it began on December 21, and in 2012, it began on December 9.
Second, because the Jewish calendar is lunar, certain adjustments must be made to the calendar to avoid having months occurring in different seasons. There are almost twelve and a half lunar months in a solar calendar year which makes a lunar calendar about eleven days too short (a thirteen-month calendar would be nineteen days too long). To compensate, the Jewish calendar uses a leap month.
Third, Hanukkah seems to be on a different day of the calendar every year because it is a festival of lights that lasts eight days and nights. The reason it lasts eight days is because in ancient times, when the Jews recaptured the second temple, there was only enough oil to keep the eternal flame lit for one day. The Jews recaptured the temple during the Maccabean revolt of the 2nd century before the Common Era. But, the flame lasted the eight days that it took to gather additional oil. To celebrate the holidays, Jews light candles on a candelabra called a menorah. On the first night, they light one candle using the Shamas, which is the master candle. On the second night, they light two candles. On the third night, they light three candles, and so on until the eighth night. On the eighth night, they light all eight candles with the Shamas.