Roman commoners are busy celebrating the Feast of the Lupercal as well as Caesar's return home. The commoners clearly favor Caesar over the defeated Pompey as Caesar was generous to the lower classes. He supported the working class by providing them extra food/money from his own pockets.
There were actually two events being celebrated, and it was the celebration itself that began the tension in the play. In Act I Scene I, the citizens were celebrating Julius Caesar's defeat over Pompey's sons after Pompey's death. Normally it was not customary to celebrate military triumphs unless it was over a foreigner. To celebrate the triumph over a fellow citizen was not appropriate. However, the people chose to celebrate anyway because they enjoyed having fun. It was more of an excuse to get out of work. This celebration of the commoners caused a split within Rome, those supporting Caesar and those against Caesar. In Act I Scene II, Caesar, his friends, and officials are celebrating the festival of the Lupercalia.