Between November 17 and 21, those who order a large black burger meal deal between 2pm and 11pm get to stuff their faces with as many burgers, fries (or onion rings) as they can for thirty minutes after ordering.
Once you finish the meal you paid for, all you need to do is go to the register with your burger wrapper, paper bag, empty cup (free refills!), and receipt to get all the Whoppers, medium fries (or onion rings), and medium coke you can handle in half an hour.
The name of this is event is "'B'iKing", a word play on the Japanese word "viking" (バイキング), which means all-you-can-eat buffet, and "Burger King".
From November 22 to November 30, the burger buffet madness can be had (enjoyed?) if you order one of the various types of large Whopper meals available at Burger King in Japan.