Final answer:
Marcus Junius Brutus, along with Gaius Cassius Longinus, was the leader of the conspirators against Julius Caesar the day before his assassination on the Ides of March in 44 BCE.
Step-by-step explanation:
The leader of the conspirators the day before the assassination of Caesar was Marcus Junius Brutus, who along with Gaius Cassius Longinus, led the group of senators who plotted to kill Julius Caesar on the Ides of March (March 15) in 44 BCE. This historical event marked a significant turning point in Roman history and led to further turmoil in the Roman Republic, eventually culminating in the rise of Caesar's heir, Octavian, later known as Augustus, who became the first Roman Emperor. While the assassination was meant to restore the Republic, it instead led to the establishment of the Roman Empire.