That's when the Mars Phoenix lander is scheduled to touch down on the red planet, today. The lander's mission is to study the Martian pole, specifically with respect to water and to search for signs of past life on the planet which is the best candidate for human colonization.
The event will surely be televised, but NASA TV will also show events on the web, some in real time and some on delay. For the socially inclined, as well as the furries and other perverts, the landing will be shown in real time on Second Life.
Via CIO, which has much more information about NASA's presence on the web for this and other missions.