Another element of the conspiracy theory that the state presented at the trial was the so-called Monday Night Meeting in the basement of Greif's Hall on Lake Street. This meeting included a few dozen (the precise number is unknown) of the more radical anarchists associated with the Northwest Side Group, among them future Haymarket defendants George Engel and Adolph Fischer. At several of these gatherings, the anarchists discussed policies and tactics in anticipation of more clashes between workers and police. No one doubted that such clashes would occur.

At another meeting the previous day these militants agreed to convene at Greif's in emergency session if they saw the letter "Y" posted in the Briefkasten (i.e., "Letter Box"), the notices column of the Arbeiter-Zeitung. Such a notice, with the additional words "Montag Abend" ("Monday evening") appeared that afternoon. The state contended that this signal was part of the Haymarket plot. Spies, the editor of the paper, claimed he had nothing to do with its inclusion and had no idea why it was inserted.

It is impossible to tell precisely what transpired at the meeting, since the fullest account is by Gottfried Waller, a Swiss-born cabinet-maker and member of the Lehr- & Wehr-Verein, one of the handful of anarchists who testified for the state in order to escape prosecution. Waller stated that he chaired the meeting, but that it was Engel who proposed a multipronged terrorist campaign prescribing what to do when trouble broke out. This included storming and bombing North Side police stations after cutting their telegraph wires.

Waller also stated that the anarchists agreed to call a massive protest rally in response to what had happened at McCormick's. The anarchists decided that this meeting should take place the next evening in the Haymarket. Adolph Fischer was assigned to secure speakers and to prepare the broadsides that would advertise the rally. In his remarks to the court before the judge pronounced sentence (included in the "From the Archive" section of Act IV), Fischer said that the Haymarket meeting was Waller's idea. Waller's full testimony is in the Haymarket Affair Digital Collection.