Obama offers gratitude to Nelson Mandela at White House tree lighting

US President Barack Obama presided over the White house Christmas tree lighting ceremony, while Mariah Carey and Aretha Franklin headlined the entertainment. "This year we give a special measure of gratitude to Nelson Mandela," Obama said at the lighting Friday, which was hampered by rain and wind. Train, Janelle Monae and Arturo Sandoval also performed at the ceremony on the South Lawn. "We're going to start at 5. It's a little wet out there, so we shouldn't start at 10," Obama said in preparing the crowd for the countdown to the tree lighting. First lady Michelle Obama appeared on stage to read "'Twas the night before Christmas" with Sesame Street character Abby Cadabby. A group of children joined them on the stage, wrapped tightly in hats and coats to protect them from the driving rain.