Brutus betrayed Julius Caesar on the Ides of March in 44 BCE.
It is a little unclear who started the conspiracy to assassinate Julius Caesar originally, or when it started. Historians’ consensus is that it was not planned very far in advance, because there were so many people involved and no serious leaks.
Marcus Junius Brutus had a complicated relationship with Caesar, mostly related to the fact that it is fairly well established that his mother and Caesar were lovers. Some historians believe Caesar may have been Brutus’s father, but most believe Brutus was too old to reasonably be Caesar’s son. He was born in 85 BCE, fifteen years after Caesar's birth in 100 BCE.
The historians also do not believe Brutus was the original leader of the conspiracy. He is usually considered a bit of a weakling by historians. Shakespeare takes this weakness and makes it idealistic and noble. Brutus was a bit wishy-washy. He followed Pompey, not Caesar, in their civil war. He then asked Caesar for clemency, which he granted.
Caesar was stabbed 32 times by many senators. The actual number is unclear, but Brutus seems to have been last. Supposedly, Caesar covered his face when Brutus went to stab him because he couldn't believe someone he trusted so much would betray him.
Regardless of who started the conspiracy, Brutus did become an important part of it. He was considered an influential figure because he had an old noble name. It may well have been Cassius who involved him, as Shakespeare depicts in his play Julius Caesar. Either way, Brutus and Cassius became the leaders of the assassins and their armies were together after the assassins went on the run.
Brutus and Cassius both committed suicide within a short time of each other. Cassius committed suicide on October 3, 42 BCE. Brutus killed himself on October 23, 42 BCE. It is also a bit unclear what happened to his body, but legend has it that he was decapitated and superstitious sailors threw his head overboard.