Today is the Ides of March, the 15th of this month (each month has Ides on various dates). While we look at the Romans as the greatest engineers and organizers until the 18th century, we then jump to the conclusion that they were rational in an Enlightenment kind of way. They weren't. Rather, they were kind of like a combination of Nuclear-Reactor-Brainiac and Management-By-Ouija-Board*.
The Ides themselves are a great example of this second thing. Their months didn't really start at day 1 and count up the same way that our months do. Well, they kind of did, but you need to break out your Ouija Board to really understand things. The Romans didn't count forwards, they counted backwards from three fixed points in the month: the Nones (usually but not always the 5th of the month), the Ides (usually the 13th but was the 15th in March, May, July, and October), and the Kalends (the 1st of the following month).
The Ides were sacred to Jupiter, Greatest and Best, and so this was a solemn day. Thus in Shakespeare we hear Julius Caesar warned to beware the Ides of March. Plutarch wrote that on his last ill-fated journey to the Senate house, Caesar saw the soothsayer who had warned about the day. Caesar joked, "The Ides of March are come;" the soothsayer is said to have replied "Aye, but not gone." Shakespeare cribbed a lot from the ancient authors.
But not everything. The famous quote from Shakespeare's play from the scene where the Senators are stabbing him is when he sees the young Marcus Brutus - of whom Caesar was very fond - among the attackers. Shakespeare has Caesar speak the words Et tu, Brute? Even you, Brutus?
Except that's not what the ancients said about that event. Suetonius says that Caesar spoke in Greek - Kai su, teknon? This was a very common expression among educated Romans, and was often used in various plays but Kai su is generally translated as "You too". That was the formal translation but it was commonly used as "Screw you". "Teknon" is generally translated as "child" but also was understood as "punk".
So Julius Caesar basically was telling Brutus to get bent. At least if you believe Suetonius.
* This is one of the things that make the ancient Romans endlessly fascinating.