Original: The first known instance of Rickroll occurred in May 2007 on /v/, 4chan's video game board, where a link to the Rick Astley video was claimed to be a mirror of the first trailer for Grand Theft Auto IV (which was unavailable due to heavy traffic).
Simplified: The first instance of rickrolling happened in May 2007 on the 4chan video game board, where a link to the video was said to be a copy of the first trailer for Grand Theft Auto IV.