It is the feast of Lupercal, a celebration of fertility in Ancient Rome, and Mark Antony won a laurel crown in the annual foot race. Casca witnesses Mark Antony attempting to offer the crown to Julius Caesar, indicating that he should be crowned king. Caesar, knowing that the people do not wish to have a monarchy, refuses to accept the crown from Antony, yet Casca tells Cassius and Brutus that he was sure that Caesar really wanted it. Three times Antony offered the crown, and three times Caesar refused, each time, according to Casca, less enthusiastically than the time before. It is Cassius' and Casca's belief that Caesar wants to make himself king of Rome. Caasca's report is filled with sarcasm and loathing.
Casca tells the events that occurred following the races. Antony offered Caesar the crown three times and watched as Caesar denied it all three times, although each time a little more gently than the last. Following this excitement, Caesar had one of his epileptic fits in front of everybody. The offering of the crown solidifies Brutus's concerns about Caesar - eventually becoming his cause for joining the conspiracy.
Casca shows early and often that he is more than willing to take part in a conspiracy against Caesar. He mentions offhandedly that if Caesar offered the people his life, Casca would be more than happy to take it from him. Casca's seeming aloofness and blunt manner suit him perfectly as an ally for Cassius.