Last year Blizzard confounded us when they sent us all a battle pet instead of a reputation gain for their tenth anniversary. But this year the Celebration Package is back.
For WoW’s 11th Anniversary, log in and check your mail. You’ll find a gift containing, among other things, a Celebration Package granting a one hour buff giving 11% bonus experience and reputation for kills. The item is not consumable and you can use it as many times as you wish during the event.
This year, the holiday runs from November 16 to November 30, so you have two weeks to grind as far as you can go. The Celebration Package will disappear after the event ends.