Brother's Day is a futuristic terrorist organization in the DC animated TV series The Zeta Project. This group was created to stop robots and synthoids from taking over the world.
Brother's Day wants to prevent an uprising from the machines, mainly synthoids. They smuggle weapons in toys that they claim to be giving to children as a charity.
One of their biggest plots was to blow up the government's biggest synthoid lab called the Gnosis. They threatened Marcus Edmund (one of the creators of Zeta) into posing as another creator of Zeta, Eli Selig to get them the coordinates of the base and then fool the perimeter defenses so they can get in.
Throughout The Zeta Project and Batman Beyond, the NSA agent Bennet thinks the Zeta is really working for Brother's Day even though he isn't.