Season One ( Book 1 : Water ) of Avatar : The Last Airbender had 20 episodes and they were shown from 21 February 2005 until 2 December 2005 . Season One ( Book 1 : Water ) of Avatar : The Last Airbender , an American animated television series on Nickelodeon , aired 20 episodes from February 21 , 2005 to December 2 , 2005 .