The Brewfest Diet is related to the seasonal event Brewfest. Completing this achievement is also an objective of the meta achievement .
This food is purchased by the <Sausage Vendor>, <Cheese Vendor>, and <Bread vendors> during Brewfest. Note that they only last 2 days, and will then be removed from your inventory. However they are not BoP, and will not be removed if you mail them to an alt and keep them in your mailbox.