The ides is the thirteenth or fifteenth of the month in the Roman calendar. When Roma Antiqua followed a lunar calendar, the ides occurred at the appearance of the full moon. However, after the calendar of Roma Antiqua had undergone numerous alterations over the years, the ides became the thirteenth or fifteenth day of the month instead.
The Ides fall on the thirteenth of January, February, April, June, August, September, November, and December.
The Ides fall on the fifteenth of March, May, July, and October.
History of the Ritual
On this day in Roma Antiqua, the FLamen Dialis made a sacrifice of a white sheep to Jupiter Optimus Maximus on the Capitoline Hill at the Temple of Jupiter Optimus Maximus.
In the Roman home, the Lares of the familia were especially honored on this day.
vindicat Ausonias Iunonis cura Kalendas;
Idibus alba Iovi grandior agna cadit;
Nonarum tutela deo caret. omnibus istis
(ne fallare cave) proximus ater erit.
omen ab eventu est: illis nam Roma diebus
damna sub averso tristia Marte tulit.
haec mihi dicta semel, totis haerentia fastis,
ne seriem rerum scindere cogar, erunt. 
The worship of Juno claims our Italy’s Kalends,
While a larger white ewe-lamb falls to Jupiter on the Ides:
The Nones though lack a tutelary god. After all these days,
(Beware of any error!), the next day will be ill-omened.
The ill-omen derives from past events: since on those days
Rome suffered heavy losses in military defeat.
Let these words above be applied to the whole calendar,
So I’ll not be forced to break my thread of narrative. 
- ↑ Ovid Fasti, c. 1 ln. 55 - 62 
- ↑ Ovid Fasti Book 1, A. S. Kline trans. http://www.tkline.freeserve.co.uk/OvidFastiBkOne.htm