\begin{definition}[Definition:Calendar/Julian]
The '''Julian calendar''' is the calendar which was introduced by [https://en.wikipedia.org/wiki/Julius_Caesar Julius Caesar] in $45 \, \text{BCE}$.
It divides the year into:
:$365$ days for every $3$ out of $4$ years
and:
:$366$ days for every $1$ out of $4$ years, known as a leap year.
The years themselves are given an index number, and are known by that number.
A standard numbering system was introduced by [https://en.wikipedia.org/wiki/Dionysius_Exiguus Dionysus Exiguus].
He identified a particular year as being $525 \, \text{A.D.}$, where $\text{A.D.}$ is an abbreviation for '''Anno Domini''', Latin for '''in the year of the Lord'''.
The year $1$ was conventionally supposed to identify the year of the birth of [https://en.wikipedia.org/wiki/Jesus Jesus of Nazareth], although the accuracy of this has since been questioned.
Years before $1 \, \text{A.D.}$ are counted backwards and assigned the label $\text{B.C.}$.
However, note that the year immediately prior to $1 \, \text{A.D.}$ is $1 \, \text{B.C.}$, not the intuitive '''year $0$''', a discrepancy that can cause confusion.
Using this system of numbering, a leap year is identified by this number being divisible by $4$.
The $365$ or $366$ days in the year are divided into $12$ approximately equal sections called months, which are assigned both names and index numbers:
{{begin-eqn}}
{{eqn | l = 1:
      | o = 
      | c = January
      | cc= $31$ days
}}
{{eqn | l = 2:
      | o = 
      | c = February
      | cc= $28$ days, or $29$ days in a leap year
}}
{{eqn | l = 3:
      | o = 
      | c = March
      | cc= $31$ days
}}
{{eqn | l = 4:
      | o = 
      | c = April
      | cc= $30$ days
}}
{{eqn | l = 5:
      | o = 
      | c = May
      | cc= $31$ days
}}
{{eqn | l = 6:
      | o = 
      | c = June
      | cc= $30$ days
}}
{{eqn | l = 7:
      | o = 
      | c = July
      | cc= $31$ days
}}
{{eqn | l = 8:
      | o = 
      | c = August
      | cc= $31$ days
}}
{{eqn | l = 9:
      | o = 
      | c = September
      | cc= $30$ days
}}
{{eqn | l = 10:
      | o = 
      | c = October
      | cc= $31$ days
}}
{{eqn | l = 11:
      | o = 
      | c = November
      | cc= $30$ days
}}
{{eqn | l = 12:
      | o = 
      | c = December
      | cc= $31$ days
}}
{{end-eqn}}
Thus, for example, the day following the $31$st of January is the $1$st of February, and the $30$th of June is followed by the $1$st of July.
\end{definition}