IDES is often the answer to the crossword clue
[Midmonth date] or [10/15, e.g.]

The “EID MAR” denarius featuring Brutus. See description below. Image via cngcoins

What’s the IDES? Easy: the [15th of March]. Everyone knows that that [March date] was a [Bad day for Casar]. And anyone–once realizing it’s not a math question–could see that it’s [3/15 but not 1/5].

Easy-peasy! (Dusts off hands, looks forward to spending the rest of the day [Killing time?])

But wait, what’s that? Even for crossword solvers it’s a [Big date] and a [Dangerous day]? And it’s [Time to beware] that the IDES isn’t just March 15, but is actually [The 13th of several months]?

[Gore’s time in the senate?]

Harold Jones, Wall Street Journal – January 30, 2016

It’s true. The IDES were monthly [Roman times], sometimes observed on [A 15th] but also [September 13, e.g.].

Since an IDES occurred on [January 13 in old Rome]–also described as [One 13th?]–we had one ten days ago. In fact, [The inauguration of a U.S. president happens a week after one].

Until I started doing crosswords and came across clues like [13th or 15th], I’d no idea!

which day is the Ides 15 or 13 each month
The IDES occur on the 13th of some months and the 15th of others, thus, it’s [13th or 15th] and [May 15 but not June 15].

The word “Ides” actually arrived in English from the French ides, based on the original Latin, īdūs–itself said to ultimately find root in an Etruscan verb meaning “divide.”

The Ides were originally supposed to align with the full moon. Add to the lunacy associated with full moons to the anger manifested on the [Fateful day in 44 B.C.] [When “et tu” was spoken], and that’s two ways IDES might be be a [Day of March madness?].

Related answers: a brief Cesarean section

It was actually CASCA who was the [First to stab Caesar].

But Caesar’s [Famous last words], the [Caesarean rebuke] ETTU, was a [Question for Brutus] meaning [“And you,” to Caesar].

About the EID MAR denarius

Besides warnings and crossword answers and and Latin and a framework for our republic, the Romans left us a lot of money. That is, there’s a lot of ancient Roman tender around. So much, in fact, that you can there are many ancient coins which can be had for surprisingly little amounts of current coins/ For instance, you can usually find a banged up but still pretty sweet denarius Marcus Aurelius for less than (warning: potential rabbit hole awaits) $20.

You won’t find an Eid Mar denarius for anything close to that. As the most famous and one of the rarest of all Roman coins, you’ll have to pay a pretty penny for one. For instance, the coin featured at the top of this post sold for $120,000.

Here a description of that coin from Wikipedia, is based on a more fulsome write-up posted as part of the Encyclopædia Romana hosted by UChicago:

Issued by Marcus Junius Brutus in 43 or 42 BC. The obverse of the coin features a portrait of Marcus Brutus. The inscription reads BRVT IMP L PLAET CEST, which means Brutus, Imperator, Lucius Plaetorius Cestianus. Lucius Plaetorius Cestianus was the moneyer who actually managed the mint workers who produced the coin. The two daggers on the reverse differ to show more than one person was involved in the slaying. The cap is a pileus (liberty cap) that in Roman times was given to slaves on the day of their emancipation, or freedom from slavery. In the context of the assassination, Brutus is making it clear the killers were defending the Republic and its people from Caesar’s attempt to attain kingship. A gold aureus with the same design was also minted. Both coins are exceptionally rare.

More on the gold aureus found here in this write up of a 2019 collectible show in Long Beach, CA.