Taken from Steam
Activision will be hosting a Free Weekend for Call of Duty®: Modern Warfare® 3! Pre-load the game now and be ready to play when the free weekend starts on Thursday, April 26th at 10am Pacific Time. (Think thats 6pm UK time)
Click here to start pre-loading, or visit www.steamgames.com/about to install Steam.
The Free Weekend of Call of Duty®: Modern Warfare® 3 will let you play the multiplayer portion of the base game.