Definition
Caput jejunii (Latin: "head of the fast") is an archaic ecclesiastical and legal term for Ash Wednesday — the first day of Lent, the forty-day penitential season observed in the Western Christian church before Easter. In historical English legal contexts, the term appears primarily in ecclesiastical law and in the computation of calendar-dependent legal dates, where Ash Wednesday served as a fixed reference point within the moveable liturgical calendar.
Why It Matters in Research
Researchers encountering caput jejunii in historical legal documents — particularly pre-Reformation English ecclesiastical records, court rolls, or instruments fixing deadlines and obligations — should recognize it as a date anchor tied to the moveable feast calendar. Because Easter itself shifts each year, any legal date reckoned from caput jejunii requires conversion using a liturgical calendar for the year in question before it can be synchronized with a civil calendar date. This is a common source of confusion in dating medieval and early modern legal instruments.
The term surfaces most often in ecclesiastical court records, church court pleadings, tithe disputes, and documents governing fasting obligations or church discipline. It is almost entirely absent from post-Reformation common law sources, having been displaced by civil calendar reckoning. Researchers working in printed legal digests and reports from the Tudor period onward are unlikely to encounter it except in direct quotation from earlier ecclesiastical instruments.
No reported cases in the Anglo-American common law tradition turn on the independent legal significance of caput jejunii as a legal concept. Its function in legal records is calendrical, not doctrinal.
Historical Dictionary Support
Black's Law Dictionary defines caput jejunii tersely as "the beginning of the Lent fast, i.e., Ash Wednesday." This is accurate and complete as far as it goes. Historical legal dictionaries offer no substantive disagreement on the meaning; the term is sufficiently technical and narrow that it does not generate interpretive controversy.
What the historical dictionaries do not address is the practical challenge caput jejunii poses in archival and documentary research: because it is a moveable date, it cannot be read as a fixed calendar anchor without additional reference material. Legal dictionaries of the Black's tradition define the term but provide no guidance on calendrical computation, which is where the term's research significance actually lies.