Why have commoners gathered in the streets at the opening of Act I, Scene I?

The answer is: to honor Caesar for his victory

In Act 1, Scene 1 of Shakespeare's "Julius Caesar," the commoners gather to celebrate Caesar's return from to Rome and celebrate his military victory over his rival Pompey. In fact, the citizens of Rome have taken the day off to commemorate the occasion, which consists of a parade through the city and the exhibition of prisoners of war.