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They're celebrating Caesar's military triumphover Pompey - Murellus scolds them because he's no lover of Caesar; he was a supporter of Pompey.

Two tribunes, Flavius and Murellus, enter a Roman street, along with various commoners. Flavius and Murellus derisively order the commoners to return home and get back to work: “What, know you not, / Being mechanical, you ought not walk / Upon a labouring day without the sign / Of your profession?” (I.i.2–5). Murellus engages a cobbler in a lengthy inquiry about his profession; misinterpreting the cobbler’s punning replies, Murellus quickly grows angry with him. Flavius interjects to ask why the cobbler is not in his shop working. The cobbler explains that he is taking a holiday from work in order to observe the triumph (a lavish parade celebrating military victory)—he wants to watch Caesar’s procession through the city, which will include the captives won in a recent battle against his archrival Pompey.

Murellus scolds the cobbler and attempts to diminish the significance of Caesar’s victory over Pompey and his consequent triumph. “What conquest brings he home? / What tributaries follow him [Caesar] to Rome / To grace in captive bonds his chariot wheels?” Murellus asks, suggesting that Caesar’s victory does not merit a triumph since it involves no conquering of a foreign foe to the greater glory of Rome (I.i.31–33). Murellus reminds the commoners of the days when they used to gather to watch and cheer for Pompey’s triumphant returns from battle. Now, however, due to a mere twist of fate, they rush out to celebrate his downfall. Murellus scolds them further for their disloyalty, ordering them to “pray to the gods to intermit the plague / That needs must light on this ingratitude” (I.i.53–54).

i'm currently reading Julius Caesar for English class right now, but I'm not sure why they were celebrating or why Marcullus scolded them.. i think they might have been celebrating Caesar's arrival.. sorry I would help you but I don't have my English book with me.

try going here: http://www.enotes.com/jc . there is a review of the story there. hope i helped :)