WoWDB

Brewfest is Azeroth's version of the real-life Oktoberfest that is celebrated in Germany. The very first Brewfest took place in 2007. Every year or two (or three...), a new commemorative Brewfest stein is introduced for that year/those years.

One of the best reasons to participate in Brewfest - besides the beer, pets, chances for mounts, and (relatively) easy epics - is the tapping of the keg which occurs at 6:15 AM and 6:15 PM Pacific (GMT-8). Vol'jin and High Tinker Mekkatorque tap the keg at the Orgrimmar and Ironforge Brewfest grounds, respectively. Players who attend the ceremony receive  Brewfest Enthusiast, which increases experience from slaying monsters by 10% for two hours. (The buff's timer does not count down while the character is logged out and it persists through death.)

There's also the  Swift Brewfest Ram and  Great Brewfest Kodo to consider, too. ;)
(Note:  It is possible to loot both mounts from a single  Keg-Shaped Treasure Chest but the chances of that occurring are extremely low.)

For a thorough guide to the world event, read Icy-Vein's guide.