Cassius is fully committed to the task at hand and he will do whatever is necessary to get it done. We can see all throughout the play. For example he coaxes Brutus through words, and even forges letters to make Brutus think that this is what the people want.
On the night before, he feels that the task will go forth well, as he now knows that he has the support of Brutus. Brutus was important to him, because the people looked up to him as a leader and one who was noble. So, with Brutus on his side, he now has traction.
On the day of the plot, there was some doubt, as Caesar's wife wanted him to stay home, but the conspirators prevailed.