Definition
De Anno Bissextili (Latin: "of the leap year" or "of the bissextile year") is the title of a medieval English legal instrument, issued in the twenty-first year of the reign of Henry III, directing the justices of the Common Bench on how to count the extra day produced by a leap year when calculating one-year time periods in legal proceedings. Though styled as a statute in historical sources, the instrument functioned in practice as an administrative writ or directive rather than formal legislation. Its practical application arose most frequently in connection with the essoin de malo lecti — a procedural device allowing a litigant confined by illness to delay appearance in court — and similar proceedings in which a party had been assigned a return date exactly one year hence. The question the instrument addressed was narrow but procedurally consequential: when the intervening calendar year contained a bissextile day (February 29), did that extra day extend the reckoning period, and if so, how?
Common Confusion
The instrument is consistently described in historical legal dictionaries as a "statute," but researchers should not treat it as legislation in the modern sense. De Anno Bissextili is better understood as a judicial directive or administrative order emanating from the crown to the bench, instructing judges on a technical counting rule. It created no general legal right and established no broadly applicable rule of substantive law. The tendency to call it a statute reflects the looser usage of that term in early English legal history, when the boundary between royal mandate and enacted law was far less defined.
Why It Matters in Research
For most researchers, De Anno Bissextili will appear as a citation artifact — a Latin heading in period sources that looks like substantive law but functions as a narrow procedural footnote to medieval court administration. Several points are worth holding:
First, the instrument belongs to the procedural world of the common law essoin system, which was already archaic by the time of Blackstone and effectively extinct in modern practice. A researcher encountering it in a 13th- or 14th-century pleading record should treat it as a signal that time-calculation rules in that proceeding are being resolved by reference to this directive, not by any enacted statute on the books.
Second, the bissextile-day problem it resolves — how to count a calendar anomaly within a fixed legal period — is a precursor to a class of problems still encountered in modern statutory and contractual interpretation: how leap-year days interact with anniversary-based deadlines. Modern legal systems address this through rules of civil procedure, contract interpretation canons, and occasionally by statute, but the underlying counting question has not disappeared.
Third, De Anno Bissextili illustrates a broader pattern in early English legal history in which royal directives to specific courts were later classified, somewhat imprecisely, as statutes or ordinances. Researchers working in Statutes of the Realm or related compilations should apply the same scrutiny to similar instruments of that era.
Finally, the term will surface in historical legal dictionaries across the Law Mind corpus under multiple headings — occasionally under "bissextile," occasionally under "anno," and occasionally cross-referenced under essoin-related entries. Tracking it requires attention to the Latin ordering conventions of the source dictionary.
Historical Dictionary Support
Black's Law Dictionary (both editions) and Burrill's Law Dictionary are in close agreement, nearly verbatim, on the substance of this entry. All three describe De Anno Bissextili as the title of a Henry III instrument, note its quasi-statutory character while acknowledging it is better understood as a judicial directive, and anchor its practical operation in the essoin de malo lecti context. None of the three sources expands on the instrument's precise operative language or explains how the extra day was ultimately to be counted — the dictionaries identify the problem the instrument addressed without resolving the counting rule for the reader. This is a genuine gap. Researchers needing the operative rule itself must go beyond dictionary sources to the instrument as reproduced in early English legal compilations or secondary treatments of medieval pleading practice.
Burrill's entry, consistent with his dictionary's general approach, follows Black's framing without independent elaboration. There is no meaningful divergence across sources on this term.
Jurisdictional Note
De Anno Bissextili is exclusively a creature of early English common law with no direct counterpart in other legal systems. It has no application in modern U.S., Canadian, or Commonwealth jurisdictions except as a historical reference point in research on medieval procedure.