From April 24th through May 8th, players will be able to enjoy the time-honored traditions of Guild Wars' birthday celebrations: Players can stroll down the Shing Jea Boardwalk for some festival games, race Rollerbeetles, unlock birthday hats, and collect some sweet, sweet treats. In addition, a new pet is being made available. The Hound of Balthazar can be unlocked for free by visiting Ceira, Sworn to Fire in Embark Beach and then traveling to the Zaishen Menagerie and tracking down the fiery beastie. After the anniversary has drawn to a close, the Hound will still be unlockable, but at the cost of 100,000 Balthazar Faction or 20 golden zaishen coins and 100 gold -- so maybe don't dawdle if you're interested in a shiny new pet. The updated page also promises "drops and activities never before seen in anniversary celebration," so join in the fun to find out what's new!
[Update: According to a Gamebreaker TV special, all of the normal weekend events observed in Guild Wars will be running simultaneously for the anniversary celebration!]