WoW Anniversary Buff Disappeared

If the buff disappears after using the Celebration Package, try resetting your interface and using the item again. If you no longer have the Celebration Package, you can purchase another from Historian Ma'di located in Caverns of Time.

Note that all the items in the Anniversary Gift are temporary and will only last as long as the Anniversary Event is active. You can check if the event is still active by looking at your in-game calendar. Once the event ends, the items will be removed from your inventory.