This is not a follow up to the B day but refers to a gag as in the kinky bondage item meant to prevent speaking. It's worn by submissives who want to hand control over to their dominants. I'm all for the gag, blindfold and restraints, and the only thing that I can't give up is hearing. There's something disorienting about not being about to hear what's going on around you even if you can feel it. 
So anyway, back to the gag. There are a few different types, with the ball gag being the most common. There's also a leather one sold in adult toys stores with a circular opening that makes breathing easier. Gags can also be fashioned with tape or other around the house items. If you decide to play, just be sure you can trust your dominant since you won't be able to use a safe word!