Hanukkah begins in the Hebrew month of Kislev.
Its first day is the 25th of Kislev, according to the Hebrew lunisolar calendar. Because the Jewish day begins at sunset, Hanukkah starts on the evening before the corresponding daytime date on a civil calendar.
Hanukkah commemorates the rededication of the Second Temple in Jerusalem after the Maccabean revolt against the Seleucid Empire in the second century BCE. The holiday lasts eight nights, with one additional candle lit on each successive night of the hanukkiah.
A common mix-up is assuming Hanukkah always falls in December. Its Gregorian dates shift because the Hebrew and Gregorian calendars follow different systems; Hanukkah can begin in late November or December. It usually ends in Tevet, the Hebrew month following Kislev.