Children from the after-school program at the 14th Street Y gathered around as Stephen Hazan Arnoff, the center’s new executive director, explained the tradition of Hanukkah before lighting the first candle with the shamash, which is used to light the others. Hanukkah, the Festival of Lights, starts on the 25th day of the Jewish calendar month of Kislev, sundown on Dec. 4, and lasts eight days and nights. Hanukkah celebrates the triumphs — both religious and military — of ancient Jewish heroes.