Judgement Day is the day that Skynet becomes self aware, starts a nuclear attack on America and the rest of the world to wipe out the human race.  In Terminator: the Sarah Connor Chronicles that date is today April 19th 2011.

If you recall from the television series, Cameron tells Sarah that Judgement Day is April 21, 2011 - however Skynet is set to go early on April 19th.    The exact quote from the Cyborg called Cameron " The Skynet missile defense system goes online April 19, 2011. Declares war on mankind and triggers a nuclear apocalypse two days later."

In the Terminator movies, Judgement Day has been altered many times.  In the first Terminator movie it was in 1997,  then in 2003, 2004, and 2005.  The date still continues to be altered because both the Resistance and Skynet are actively seeking to interfere with the timeline and are having unforeseen effects. 

During Judgement Day these are the cities that are destroyed. Amsterdam, Athens, Atlanta, Austin, Baghdad, Baltimore, Bangkok, Beijing, Berlin, Boston, Brisbane, Brussels, Cairo, Canberra, Casablanca, Columbus, Dallas, Denver, Detriot, Dubai, Houston, Istanbul, Jakarta, Johannesburg, Karaci, Kiev, Lagos, Las Vegas, London, Los Angeles, Madrid, Manila, Mexico City, Montreal, Moscow, Mumbai, Nairobi, New York, New Orleans, Osaka, Paris, Perth, Phoenix, Rio De Janiero, Riyadh, Rome, Saigon, San Antonio, San Diego, San Francisco, San Jose, San Paulo, Seattle, Seoul, Shanghai, St. Petersberg, Russia, Sydney, Taipei, Tehran, Tokyo, Toronto, Warsaw, Washington DC, Wodonga - so for now the Capital Region on New York is safe, however there will be nuclear fallout.