The Hebrew calendar (which is also referred to as the Calendar of the Jews or Jewish calendar) is actually based on three certain astronomical phenomena. The astronomical phenomena in particular are the Earth revolving around the sun which constitutes a year. This is the same basis as the solar calendar. The second phenomenon is the revolution of the moon about the Earth which constitutes a month. Also known as a lunar cycle that constitutes a lunar month, this phenomenon plays predominately throughout the Hebrew calendar. It serves as the basis for many of the celebrations and festivities in Jewish culture. Finally, the third phenomenon is the rotation of the Earth about its axis. In layman’s terms this is called a day. A day is achieved when the earth completes a full rotation on its axis.
Strangely enough all three phenomena are independent of each other. The time it takes the moon to revolve around the Earth is roughly twenty nine and a half days. The Earth then takes roughly three hundred sixty five and one fourth days to revolve around the sun. This is approximately twelve plus lunar months.
The civil calendar that most people are probably more familiar with and what most of the world uses, has discarded any correlation between the cycles of the moon and what constitutes a month. It sets the length of months to either twenty eight, thirty or thirty one days.
The Hebrew calendar, on the other hand still coordinates all three of the astronomical phenomena. The months are either twenty nine or thirty days. This corresponds to twenty nine and a half day lunar cycle. A year in the Hebrew calendar is composed either twelve or thirteen months. Again this is in accordance to the 12.4 solar months or one solar cycle.
A lunar month on the Hebrew calendar is based on the different phases of the moon. When the first crescent or sliver of the moon becomes visible after a dark of the moon phase, a lunar month has begun. In early Hebrew, determining the new months was based on pure observation of the lunar phases. When people observed a new moon, they would notify the Sanhedrin. The Sanhedrin was the highest judicial and ecclesiastical or religious council of the Jewish nation in ancient times. They are composed of from 70 to 72 members. When the Sanhedrin heard the statement from two independent, reliable individuals who witnessed the creation of the new moon its occurrence on a certain date, they would then declare that date the rosh chodesh. This is what they referred to as the first of the month. Messengers would be sent out to the passes informing them of the rosh chodesh and when it occurred.
The main problem with a lunar calendar, to which the Jewish Calendar is based heavily upon, is that there are 12.4 lunar months in every solar year. So, a twelve month lunar calendar is actually eleven days shorter than a solar year while a thirteen month lunar calendar is roughly nineteen days longer than a solar year. The months thus tend to drift around the seasons on such a calendar. An example of this would be in a 12 month lunar calendar, the month of Aviv or known as Nissan which “spring” in Hebrew would occur earlier in the season than usual each year. It would go on like this until eventually; it would have the start of the “spring” month occurring during winter. The reverse would happen if it occurred during a 13 month lunar calendar. To compensate for this gradual shift, the Hebrew calendar uses a 12 month calendar with an extra month added every two or three years. Case in point, the month of Aviv occurs eleven days earlier each year for two to three years, and then jumps forward by thirty days, once again realigning the entire calendar with the planetary seasons.
In the earlier days of the Jewish people this additional month composed of thirty days was also based on observation. The Sanhedrin observed the conditions of the weather, the current condition of crops and livestock cycles and if they were not suitably ready to be considered “spring” then the council would insert another month into the calendar to guarantee that that the religious celebration of Pesach which is known in the Christian realm as the Passover would occur in the spring. In the Hebrew book the Torah, the Pesach is referred to as the Chag he – Aviv, or commonly known as the festival of spring.
On the other end, a thirteen month Hebrew calendar would be considered a leap year in a Western Calendar. Unlike the Western calendar where a leap year only calls for an additional day in the month of February, the Shanah Me’uberet (which literally translates to: a pregnant year) is composed of an additional month. This additional month, Adar Rishon was named Adar I or the first Adar, or Adar Alef (In Hebrew letter Alef would represent the numerical “1”). The added month of Adar I is declared prior to the standard month of Adar, known during the leap years, Adar Beit or Adar Sheini as Adar II. For those who are not familiar with the Jewish calendar, during the Shanah Me’uberet Adar II is the “real” Adar or the Adar during normal years. Adar I is the additional Adar and ONLY appears during the Shanah Me’uberet.
By the fourth century, Hillel II a Jewish communal and religious entity around 330-336 CE established a fixed calendar based on the on a combination of mathematical and astronomical calculations. This calendar, which is still recognized and applied today, standardized the length of months and the addition of months over the course of a 19 year cycle or Metonic cycle which is 235 lunar months. This allows for the lunar calendar to realign itself with the solar calendar in years. Adar I is added in the third, sixth, eighth, eleventh, fourteenth, seventeenth and the nineteenth year of the cycle. Today, the current cycle began in the year 5758 which when converted into the Western calendar or Gregorian calendar was October 2, 1997.