Shakespeare is notorious for pliable history, that which he can bend, stretch, mold and work into any shape which pleases him and helps him make a dramatic (and–if you’re inclined–political) point. Remember Hotspur-as-contemporary-of-Hal (when he was actually older than Henry IV)? Or the ridiculous Tudor-centric retelling of the War of the Roses stuff in the Henry VI plays? Well, there’s nothing that bad in Julius Caesar, but there is some fudging of facts…
Most of the issues are purely for pacing/dramatic tension reasons.
The play opens with a combination of Caesar’s triumph and the Feast of the Lupercal. In reality, these were two individual events, separated by many months. Caesar had returned to Rome in September of 45 BC, and the triumphs were held shortly thereafter. Lupercalia took place between the 13th and 15th of February.
The play gives the impression that the assassination, funeral oration, will-reading, and arrival of Octavius all take place on the same day. In reality, the assassination took place on March 15, the will was read three days later, and the funeral took place two days after that. And Octavius? He didn’t arrive in Rome until May.
After the death of Cassius, Brutus says that “yet ere night // We shall try fortune in a second fight” (V.iii.108-9). In reality, the Second (and climactic) Battle of Philippi took place almost three weeks after the First.
All of these push the pace of the play, driving the momentum of the piece. There are two other changes to history that Shakespeare makes that have nothing to do with pacing, however.
In the play, the assassination takes place at the Capitol. That’s a pretty dramatic setting. However, the reality, if anything, was even more symbolic. Caesar was killed at the Curia of Pompey, a meeting hall at the Theater of Pompey. Given the past conflict between Caesar and Pompey, and the personal history that many of the conspirators had with the late Pompey the Great, it’s a fitting locale. Of course, Shakespeare gets to play it both ways a little: he just happens to have a statue of Pompey in the Capitol, at the base of which Caesar’s dead body lay.
Finally, we have the issue of Caesar’s last words: “Et tu, Brutè? Then fall Caesar” (III.i.76). There’s no historical basis for this. The Roman historian Suetonius says there were unsubstantiated reports of Caesar saying, “And you, child?” The Latin and its addendum aren’t all Shakespeare’s either, though: those words were part of other Elizabethan plays that Shakespeare pilfered.