ABC confirms that Charlie Gibson will be stepping down from his post at World News Dec. 18. Executive producer Jon Banner wrote the following on his “World Newser” blog: “We’ve known this day was coming. Charlie first told us of his desire to retire in early September. As he said then, it’s the right time for him and his amazing family.” Banner also wrote that Gibson’s final week on air will focus on some of the anchor’s best work over the past three decades. In a low-key switch-over, Diane Sawyer will begin anchoring the program Dec. 21.