Definition
A fixed day within the Roman calendar used as a temporal reference point in ancient legal documents, ecclesiastical records, and instruments prepared under Roman legal tradition. The Ides fell on the 15th day in March, May, July, and October, and on the 13th day in all other months. Dates in Roman legal and ecclesiastical texts were expressed by counting forward from or backward to one of three monthly markers: the Kalends (1st), the Nones (5th or 7th), and the Ides. A document dated "the Ides of March" would thus correspond to March 15 in the modern calendar.
Common Language
Modern common usage (Wiktionary): The notional full-moon day of a Roman month, occurring on the 15th day of the four original 31-day months (March, May, July, and October) and on the 13th day of all other months.
Historical common usage (Webster's 1913): The fifteenth day of March, May, July, and October, and the thirteenth day of the other months.
The common understanding of the Ides — shaped heavily by Shakespeare's "Beware the Ides of March" — associates the term loosely with March 15 and little else. In legal and ecclesiastical contexts, the Ides operated as one component of a complete dating system applied across all twelve months. A researcher encountering an Ides date in a non-March document must apply the correct day (13th or 15th) for that specific month, a step the popular understanding does not prompt.
Why It Matters in Research
Researchers working with medieval ecclesiastical records, canon law instruments, papal documents, Roman civil law texts, or early English legal instruments influenced by Roman practice will encounter the Ides as a dating convention. The key research task is translation: converting an Ides date into a modern calendar equivalent requires knowing both the month and whether that month falls into the 15th-day group (March, May, July, October) or the 13th-day group (all others).
Two practical traps arise in historical sources. First, some instruments express dates not on the Ides itself but as a count of days before the Ides — for example, "three days before the Ides of May" — requiring subtraction from the Ides date rather than simple lookup. Second, the Roman calendar reckoning counted inclusively, meaning "three days before the Ides" in May would be May 13 (15 minus 3, plus 1 for inclusive counting), not May 12. Errors in this arithmetic are common in secondary sources.
The historical legal dictionaries uniformly note that the Ides system survived in active use in the Chancery of Rome and in the Breviary calendar well into the modern period, meaning this is not purely an antiquarian concern. Canon law documents and Vatican instruments issued through the nineteenth century and beyond may still employ this dating form.
For researchers in American or English common law contexts, direct encounters with Ides-dated instruments are rare but not unknown in materials touching ecclesiastical property, church records, or documents with continental European provenance.
Historical Dictionary Support
The four source dictionaries converge tightly on substance. All agree on the core rule: March, May, July, and October place the Ides on the 15th; all other months place it on the 13th. All cite or echo Wharton's note that the system remained in use in Rome's chancery and the Breviary calendar. Rapalje & Lawrence and both Black's editions reproduce this note almost verbatim.
Burrill's Law Dictionary adds the most useful detail for a researcher: it traces the Latin root (idus, from the Old Latin iduare, to divide) and cites Adam's Roman Antiquities for further authority on the three-part monthly division system. This etymology clarifies why the Ides functioned as a dividing marker within the month rather than a standalone date — understanding the system as tripartite (Kalends, Nones, Ides) helps researchers decode unfamiliar date expressions in Roman-influenced legal documents.
What the historical dictionaries do not address is the mechanics of counting days before the Ides — the more practically challenging problem a researcher actually encounters in source documents. For that, general reference works on Roman chronology and classical antiquities are more useful than the legal dictionaries alone.