People who believe in luck and bad luck -- that is to say , those considered superstitious – may think that some days or events are unlucky . In the United States , Friday the 13 is known as an “unlucky ” day by superstitious people . Another famously unlucky day for some is March 15 , also known as , “The Ides of March .” The Ides of March is so unlucky that it is often spoken as a warning : “Beware the Ides of March .” First , what exactly is an “ides ?” The Latin root of “ides ,” a singular word , means “to divide .” So , the ides of any month falls in its middle . The Roman calendar set ides on the 15th in March , May , July and October or on the 13th in the other months . The ides of a month usually falls on the first day of a full moon . In Roman times , the Ides of March was known as a date for paying off debts . But this connection was quickly lost because of different timing between calendar months and changes in the moon's appearance . The Ides of March was also a time people celebrated Mars , the god of war . But why do some people still use the expression , "Beware of the Ides of March "? Why do they consider it an unlucky day ? March 15 was very unlucky for one man a very long time ago . The British playwright William Shakespeare takes this incident from Roman history and freezes it forever in literature . In Act 1 , Scene 2 of "Julius Caesar ,” Shakespeare writes about a meeting between the dictator and a “soothsayer ,” someone who can predict the future . “Beware the Ides of March ,” says the soothsayer . Caesar asks , “What man is that ?” And Brutus , Caesar's loyal friend answers , “A soothsayer says to (bids you ) beware the Ides of March . Caesar does not listen to the warning . He should have . In Act III , Scene I , Shakespeare's famous line seems to show that Caesar is asking for trouble . Caesar says to the Soothsayer , “The Ides of March are come .” The Soothsayer answers , “Aye , Caesar , but not gone .” When he sees that Brutus has turned against him , he says , “Eh tu , Brutus ?” This means “You too , Brutus ?” This expression is widely used in Western culture when someone is betrayed by a friend . “The Ides of March ” is also the name of a U.S . film made in 2011 . George Clooney directed the film . He also stars in it as a promising presidential candidate . While no one gets assassinated , “The Ides of March ” is a film about a friend being disloyal to a friend in the world of politics . Often in politics , loyalty and friendship do not go hand-in-hand . Julius Caesar learned this the hard way on the Ides of March .