The Ides of March is just the 15th of March.

The Ides was a marker day used by the Romans to divide each month into two.

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).
Nones (the 7th day in March, May, July, and October; the 5th in the other months).
Ides (the 15th day in March, May, July, and October; the 13th in the other months).

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 didn’t signify anything special in itself. In Shakespeare’s day that just the usual way of saying “March 15th”.

The notion of the Ides being a dangerous date was purely an invention of Shakespeare’s.