15th March is known as The Ides of March in Roman tradition and the “Ides” of any month were sacred to Jupiter god of the sky and the supreme god of the Romans prior to Christianity. 15th March was also the Festival of Anna Perenna the goddess of the eternal year. 

But why is it known as the “Ides of March”

The word "Ides" stems from the earliest Roman calendar, which was supposedly devised by Romulus, the mythical founder of Rome. Whether it was Romulus or Remus or not, the inventor of the calendar certainly made it very complex by modern standards. This calendar organised its months around 3 specific days, each of which was a reference point for counting other days: These 3 specific days were known as:

• Kalends (1st day of the month)
• Nones (the 7th day in March, May, July, and October; the 5th in the other months)
• Ides (where the 15th day in March, May, July, and October; the 13th in the other months). 

The remaining, days of the month which were named were identified by counting backwards from the 3 named days. For example, 3rd March would be Nones-V i.e. 5 days before the Nones. It seems exceeding complicated to me and how they knew what day of the month it was I have no idea. 

Why do we “Beware the Ides of March”?

• “Beware the Ides of March," has always impressed the March Ide with a sense of foreboding. But in Roman times this would not have been the case, the expression "Ides of March" it was simply the standard way of saying "March 15."
• “Beware the Ides of March” and its current sense of foreboding came about because reputedly the Seer, Spurinna, told Julius Caesar to “beware the Ides of March” because harm could come to him by then. On his way to the Theatre at Pompey Caesar apparently passed the seer and joked. “The Ides of March have come, meaning the prophecy had not been fulfilled, and the seer replied “Aye Caesar, but not gone’. This is the basis of William Shakespeare’s famous scene in his play of Julius Caesar.  Caesar was in fact assassinated on 15th March 44BC.