Why is the episode 9 season 1 of "Colony" called "Zero Day"? Is this some reference to Columbine High School massacre that inspired filming a movie called "Zero Day"? Or is it related to revolution? "restarting and going all over again" in fighting with aliens?
Notably, the opening scene scrawl for this episode reads "366 days". Families are releasing sky lanterns with the faces of other people who are not there attached to them; those others are likely missing or dead. The families are mourning together.
We would assume "Zero Day" refers to the annual anniversary of the public exposure to the alien invasion; specifically, the day the walls came down, during which many were likely killed or separated.
The episode (or part of it) takes place on this "Zero Day" anniversary.