Definition:Calendar/Julian

Definition
The Julian calendar is the calendar which was introduced by 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 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 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 $1 \, \text{B.C.}$, 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:

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.

Also see

 * Discrepancy between Julian Year and Tropical Year