Disclaimer: Made with concepts derived from Shakespeare! And a bit of history. What neither of them wish to claim is, I suppose, mine. Set (very roughly) two to six months before the beginning of the play.
Cassius had, at best, mixed feelings regarding the Senate sitting on rainy days. It was not that he himself disliked the rain; indeed he immensely enjoyed most weather that kept all other men shut firmly within doors.
On those days when the rain began falling early in the morning or even before dawn, the Senate meeting might be called off for the day, or else the Senators made their way to the curia under canopies born by slaves. It was the days when rain began mid-morning, when most of the Senators were halfway between home and the Senate-house, that were by far the worst. When at last they arrived, it was to cram the curia full of half-drenched men who, in their half-drenched togas beginning to steam from the heat of several dozens of bodies packed together, smelled like so many sodden sheep milling about in a pen. In his particularly uncharitable moods, which such days always brought out, Cassius thought the analogy apt not only with respect to the nose but to temperament as well.
When Caesar had crossed the Rubicon, the Senators had gone milling after Pompey, following him to Greece in their hearts, at least (Cassius would not credit them with minds), if not with strength of arms. Cassius had gone that way too, of course, but it had been with strength of arms and not in woolen fuzziness. But now again in the wake of Pompey's death, they milled about at Caesar's heels, voting ever absurdly greater honors upon him.
That was one thing about sheep: they didn't vote, so far as Cassius had observed, though it was just the sort of thing they would do to make a wolf their leader. Cassius was not a sheep. He did vote, and he voted against the powers and the honors.