look in your web analytics. the dates that a lot of people saw sharp traffic declines are:

    Panda Update 1.0: Feb. 24, 2011
    Panda Update 2.0: April 11, 2011 (about 7 weeks later)
    Panda Update 2.1: May 10, 2011 (about  4 weeks later)
    Panda Update 2.2: June 16, 2011 (about 5 weeks later)

source: http://searchengineland.com/why-google-panda-is-more-a-ranki...