This is because Sky chose it for their Easter coverage. The game is now at The Valley on the same day, but will start at 8pm for the cameras.
That means Charlton's two games over the Easter period will both be shown on television. The away match against Manchester City on Good Friday is on pay-per-view TV, with a 5.15pm kick off.
The Reading game is the second game to be moved recently for The Addicks, with the home match against Newcastle, originally scheduled for Saturday 17th March with a 3 o'clock kick off, being moved to the Sunday of the same weekend, with a 1.30pm start, due to Newcastle's participation in the UEFA Cup.