We know rather less about the procedure of the senate than you might expect, a problem made significantly more difficult by the simple fact that there was no prescribed procedure in the Republican senate beyond common custom. This is not unique to the senate, as the procedures of Republican institutions were generally speaking not formally arranged or defined. Some scholars, like Benjamin Straumann, argue for an increasing push towards "constitutionalism" in the late first century that was snuffed out by the imperial state, but this is a controversial proposal and I'm not convinced that Straumann's emphasis on the Sullan legislation, most of which was restrictive rather than prescriptive or reformative, really holds water. The same unclarity as in the senate occurred in the assemblies, which came to define their own procedures as time went on, and it was really only in the courts (following the--probably--Gracchan establishment of standing quaestiones) that legal statutes regulated precise procedure. Even in the courts, though, the presiding judge (the quaesitor) typically had great freedom to set procedure.

By the first century the consuls, praetors, and tribunes could call for meetings of the senate, which could occur at any place within or just outside the pomerium that was consecrated as a templum. That doesn't literally mean a temple--the Curia Hostilia and the curia adjoining the Theater of Pompey where Caesar was killed were both consecrated as templa--but temples were common places for senatorial meetings, particularly the temples of Apollo and Belona on the Campus Martius and the temple of Concord in the forum. The site where a magistrate called a meeting of the senate could have symbolic significance, which is why during the Catilinarian Conspiracy Cicero called the senate to the Temple of Concord. Meetings of the senate were often announced well in advance, but at least as frequently they seem to have been sudden summons. They could not occur on the same day as assembly votes, but otherwise they could be called on festivals and other public holidays (like some of the courts but unlike the voting assemblies).

There was no formal attendance requirement for any individual senator, and senators frequently skipped meetings or were away in the Italian countryside (or of course even abroad on assignment), so there seems often to have been a requirement for a quorum of members. It's unclear how this worked. A numeration of present senators could be made on request, although I don't think there's any actual attestation of this ever occurring. Francis Ryan identifies thirty-five times in Cicero alone that the term "frequens senatus" is used, referring to twenty-six individual meetings. The general consensus is that frequens senatus is a technical term, although Cicero doesn't always use it in its strict technical sense. The precise meaning of the phrase is not entirely clear, and seems to have depended on the type of meeting called (Ryan has a full discussion in Rank and Participation in the Republican Senate, but it's not worth worrying about here). In the post-Sullan senate (which Ryan insists was fixed at 600 members, but that's not how senatorial membership worked) frequens senatus and frequentissimus senatus, the meaning of which is even less clear, seem to mean a "quorum" of the senate, and from Cicero it seems that senatorial meetings could be summoned as a frequens senatus, apparently not beginning formally unless a certain number of senators were present. This number is not clear. We have only four precise numerations of the senate, all of them from the post-Sullan period. Both the highest (417) and lowest (200) numbers of senators that we're told about at a single meeting come from meetings in 57, and both probably represent a quorum. Some laws and statutes prescribed that a certain number of senators had to be present for particular actions. So the S.C. de Bacchanalibus of 186, the earliest surviving senatus consultum, specifically says that petitioners asking to be allowed to practice the Bacchanalia should be heard by no fewer than 100 senators, which in those days would have probably been about a third of the senate on average. Asconius tells us that when Cornelius withdrew his bill transferring the approval of exemptions from the law to the assembly and resubmitted it in 67 he added in a provision that the assembly's vote on such cases should be preceded by an SC of at least 200 senators, which in 67 would have also probably been about a third of the senate.

Senatorial meetings began with the magistrate who called the meeting laying out the topic of discussion. This could be a single discussion ("Do we execute the Catilinarian conspirators?") or probably more commonly the senate would discuss a list of topics, sometimes related sometimes not. How this worked exactly is pretty obscure to us. In his relatio the presiding magistrate could call on other people to speak. So for example a meeting to discuss a foreign embassy would typically involve the presiding magistrate producing the envoys, who would plead their case. Senatorial subcommittees existed to frame particular problems or investigate issues, but we don't entirely know how they worked. The SC de Oropiis mentions the existence of a subcommittee (including none other than Cicero himself) set up to investigate the claims of the Oropian envoys. This committee produced a report, which presumably was presented during the relatio at the beginning of the senatorial meeting that resulted in the SC. Most minor or routine decisions seem to have been decided without complex debate, although we have very little information about these because they weren't significant enough for anyone to talk about them.

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