Explain why the working men are celebrating in the first scene in Julius Caesar. Why does Marulles reproach them?

The workmen are celebrating having a day off for Caesar’s celebration.

The plebeians of Rome are celebrating a holiday from work.  This holiday is a result of the Feast of Lupercal celebration that Julius Caesar is officiating.  Due to the celebration, the ordinary workers have the day off.  Flavius takes note of this. 

Hence! home, you idle creatures get you home:Is this a holiday? what! know you not,Being mechanical, you ought not walk...

The workmen are celebrating having a day off for Caesar’s celebration.

The plebeians of Rome are celebrating a holiday from work.  This holiday is a result of the Feast of Lupercal celebration that Julius Caesar is officiating.  Due to the celebration, the ordinary workers have the day off.  Flavius takes note of this. 

Hence! home, you idle creatures get you home:
Is this a holiday? what! know you not,
Being mechanical, you ought not walk
Upon a labouring day without the sign
Of your profession? (Act 1, Scene 1)

The workmen are not overly political.  They are just happy that they do not have to go to work.  Marullus and Flavius are not supporters of Caesar.  They chide the workmen for celebrating Caesar’s victory against Pompey.  Pompey was a Roman consul too, but Caesar was victorious against him in a civil war.  Marullus scolds the men for changing allegiances. 

Wherefore rejoice? What conquest brings he home?
What tributaries follow him to Rome,
To grace in captive bonds his chariot-wheels?
You blocks, you stones, you worse than senseless things!
O you hard hearts, you cruel men of Rome,
Knew you not Pompey? (Act 1, Scene 1)

Marullus and Flavius are part of a faction that opposes Caesar.  He was aggressive in pursuing the war against Pompey and the other Romans.  Some of the members of the Patrician class felt that he was overly ambitious.  This is the same reason a conspiracy of senators rose up to assassinate him in the first place.

Shakespeare uses this scene for two reasons.  First, it establishes comic relief through the workmen’s puns.  Second, it demonstrates the political unrest in Rome.  Caesar came to power and became dictator under very unusual circumstances.  Not everyone in Rome appreciated it.  This scene also gives us some background information on Pompey and those who were still supportive of him.