The expression 'Beware the Ides of March' is first found in Shakespeare's Julius Caesar, 1601. The line is the soothsayer's message to Julius Caesar, warning of his death.
The Ides of March did not signify anything special in itself - this was just the usual way of saying "March 15th". The notion of the Ides being a dangerous date was purely an invention of Shakespeare's imagination. Each month has an Ides (often the 15th) and this date was not significant in being associated with death before 1601.
Months of the Roman calendar were arranged around three named days – the Kalends, the Nones and the Ides - and these were reference points from which the other (unnamed) days were calculated:
Kalends 1st day of the month.
None's 7th day of the month in March, May, July, and October; the 5th in the other months.
Ides are the 15th day of the month in March, May, July, and October; the 13th in the other months.
Although the only time we are likely to hear 'the Ides' being used today is in a line from Shakespeare, in the Middle Ages, the Ides, as well as the Kalends, were in common usage to depict the date. There are pre-Shakesperian references in Middle English to the Ides' other months. An example is found in John Capgrave's Life St. Gilbert, circa 1451. Her lith Seynt Gilbert was translated into his shrine... þe þirde yde of October.- Here lies Saint Gilbert, interred in this shrine on the third Ide of October.
Note that the method of reckoning dates then was to name an Ide and then work back from it. So, St. Gilbert's date above would be three days before the Ide, i.e. the 13th of October.
These days the Ides of March passes by each year pretty much unnoticed.