Definition
A unit of time measurement with multiple legal meanings depending on context. Courts and statutes use "day" in at least three distinct senses:
1. Natural day (also called the civil day). The full twenty-four-hour period running from one midnight to the next succeeding midnight. This is the default legal meaning when a statute, contract, or rule uses "day" without further qualification.
2. Solar day (also called the artificial day). The period during which the sun is above the horizon — from sunrise to sunset. Relevant primarily in criminal law contexts where the distinction between day and night affects liability (e.g., burglary, lawful entry, use of force), and in older common law rules governing the service of process.
3. Business day / court day. The hours during which courts are open and business is transacted. A "day" in this sense may be considerably shorter than twenty-four hours and varies by jurisdiction, local rule, and custom.
When a deadline is stated in days — whether by statute, rule, or contract — the natural day (midnight-to-midnight) is presumed unless context or express language indicates otherwise.
Common Language
Modern common usage (Wiktionary): Wiktionary's primary legal-adjacent content for "day" focuses on surname origins, which is unhelpful here. The ordinary English sense is the period of light between sunrise and sunset, or loosely the full twenty-four-hour period.
Historical common usage (Webster's 1913): "The time of light, or interval between one night and the next; the time between sunrise and sunset, or from dawn to darkness... The period of the earth's revolution on its axis, ordinarily divided into twenty-four hours."
The gap matters in legal research. Ordinary English freely uses "day" to mean either the daylight hours or the full twenty-four-hour cycle, and context usually resolves the ambiguity. Legal usage cannot tolerate that ambiguity: a deadline that expires "in 30 days" must attach to a fixed clock, and courts have long defaulted to the midnight-to-midnight rule precisely to eliminate the ambiguity that common speech leaves open. Researchers reading historical documents must be alert to which sense a drafter intended, since pre-nineteenth-century instruments sometimes used "day" to mean only the solar (daylight) period.
Common Confusion
Day vs. calendar day vs. business day vs. court day. Modern procedural rules frequently distinguish these explicitly (see, e.g., Federal Rule of Civil Procedure 6), but older statutes and instruments often do not. A "day" in an eighteenth- or nineteenth-century statute may mean the solar day, the civil day, or the court's sitting day depending entirely on context. Researchers encountering deadline language in historical documents should not assume the modern midnight-to-midnight default applied.
Day vs. juridical day. A juridical day is a day on which courts may lawfully sit and act. Sundays, public holidays, and days of court recess are ordinarily excluded. When a limitations period or procedural deadline falls on a non-juridical day, most modern rules extend it to the next juridical day — but historical practice varied, and the exclusion was not always automatic.
Recognized Forms
/SUBTYPES
Natural day (civil day): Midnight to midnight; twenty-four hours. The standard default in modern statutory and contractual computation.
Solar day (artificial day): Sunrise to sunset. Operative in criminal law definitions (burglary at common law required breaking and entering in the nighttime), process service rules, and some agricultural and maritime contexts.
Juridical day (court day): A day on which a court is authorized to conduct business. Excludes Sundays and legal holidays unless a rule provides otherwise.
Business day: A day on which commercial or governmental offices are open. Defined variously by statute; typically excludes Saturdays, Sundays, and public holidays.
Clear days / clear calendar days: A counting convention that excludes both the first and last day of a period. Used in notice requirements and some limitation provisions. Distinct from inclusive counting methods that count both endpoints.
Why It Matters in Research
Deadline computation is among the most consequential and most error-prone areas of procedural law, and "day" is its foundational unit. Several research traps deserve attention.
Historical sources use the solar day as a baseline. Coke, Bracton, and the early common law authorities treated the solar day — not the midnight-to-midnight civil day — as the intuitive unit. Burrill and Rapalje confirm this dual tradition. Researchers reading pre-modern instruments or cases must check which sense governed; importing the modern default backward is anachronistic.
The Saturday-to-Monday problem. Bouvier flags the specific rule, codified in some jurisdictions (e.g., Kentucky Statutes § 455 as reported), that an adjournment "from day to day" from Saturday to Monday is legally valid. This reflects the broader principle that when a deadline falls on a Sunday or holiday, it rolls to the next valid day — but the rule's expression varied widely by jurisdiction and era. Do not assume uniformity across the corpus.
"Clear days" counting is a trap in historical instruments. Wills, notices, and conveyancing documents from the eighteenth and nineteenth centuries frequently specified notice periods in "clear days," excluding both endpoints. Modern rules often specify inclusive or exclusive counting explicitly; historical documents often did not, and case law resolved the ambiguity inconsistently.
Midnight birthdays and exact-moment questions. Bouvier preserves the curious English case of a child born at the stroke of midnight across competing clocks on the night of January 4–5, 1805. While anecdotal, this illustrates a genuine research problem: when legal rights attach to the day of birth, death, execution of an instrument, or filing, the precise clock rule — and which clock — can be outcome-determinative. Modern electronic filing systems have largely resolved this for court filings, but the question remains alive in estates, insurance, and contract contexts.
The corpus contains numerous deadline-specific entries (30-day letters, 90-day periods, filing windows) where the definition of "day" interacts with tolling, holidays, and jurisdictional computation rules. Cross-reference those entries before assuming any deadline is simply a raw count of calendar squares.
Historical Dictionary Support
The historical dictionaries agree on the basic tripartite structure — natural day, solar day, civil/midnight-to-midnight day — but differ in emphasis and application.
Black's (both editions) and Burrill lead with the twenty-four-hour civil day as the primary legal meaning, citing Coke (Co. Litt. 135a) and Blackstone (2 Bl. Comm. 141) for the midnight-to-midnight formulation. This reflects the consolidation of the civil day as the legal default that had occurred by the late nineteenth century.
Rapalje & Lawrence preserve the older Coke framework more faithfully, presenting natural and artificial days as coordinate categories with equal standing, and noting that in "old practice at common law, day had several technical meanings." This framing is more useful for researchers working with pre-1800 materials.
Anderson's is the most practically organized, listing the three senses sequentially and noting the specific application of "business hours of a day" as a recognized legal unit — a category the other dictionaries underemphasize.
Bouvier is the most anecdotal and jurisdiction-specific, offering the Kentucky adjournment statute and the midnight-birth puzzle. These are genuine contributions to understanding how courts resolved the ambiguities the definition creates, but Bouvier does not synthesize a clear rule.
What the historical dictionaries collectively miss: they do not address "clear days" counting with the rigor the topic deserves, and they predate the systematic procedural-rules codification (Federal Rules of Civil Procedure, 1938) that made "calendar day" vs. "business day" vs. "court day" a matter of explicit rule text rather than judicial inference. Modern researchers should treat the historical dictionaries as background on the common law baseline and consult current procedural rules for operative definitions.
Jurisdictional Note
Federal practice (FRCP 6 and analogous rules) defines "day" as a calendar day and provides explicit rules for counting, excluding the event day, and rolling deadlines that fall on weekends or legal holidays. State practice varies: some states follow the federal model closely; others retain older formulations. International instruments (treaties, commercial contracts) sometimes use "business day" referenced to a specific financial center's calendar, which can differ from both federal and state defaults.
Encyclopedia Cross-Reference
IRS Notice of Deficiency 90-Day Letter (The Law Mind Tax Encyclopedia) — for the specific operation of a statutory day-count deadline in a tax context.
Federal Court Review — Standard of Review, Exhaustion, and the 30-Day Filing Deadline (The Law Mind Immigration Law Encyclopedia) — for another example of a fixed-day deadline with jurisdictional and tolling implications.