BOCs can happen randomly during Woot-Offs (there will be one today in the Clearance section), sometimes during special events, and in the app once a week (I think).
The Woot app (on Android and iOS) runs games on the weekends (Friday through Sunday), and you can find them in the app on the main page down near the bottom. The games go live at Midnight Central Time and offer a variety of rewards (things you can by). If the BOC is offered, you have to try right away because they will sell out pretty quickly. Follow the instructions for whatever game is offered for a chance to “win” (purchase).
(Please note: I don’t work for Woot, I just volunteer to help out here on the forums.)