When did avatar first go on television?

It depends on which "Avatar" you're asking about:

* Avatar: The Last Airbender: This animated series first premiered on February 21, 2005 on Nickelodeon.

* Avatar: The Legend of Korra: This animated series is a sequel to Avatar: The Last Airbender and premiered on April 14, 2012 on Nickelodeon.

So, the first Avatar series to appear on television was Avatar: The Last Airbender in 2005.