Just before supper on Christmas Eve (where frumenty is eaten), while the Yule log is burning, all other lights are put out, and the candles are lit from the Yule log by the youngest person present.
Burning the Yule log was an ancient custom meant to honor the Sun and encourage its return after the long, dark winter nights. The Yule log was often kept burning for 12 days and nights, and its ashes were believed to bring good luck and protect the home.