The Jewish calendar is based on three astronomical phenomena: the rotation of the Earth on its axis (a day); the revolution of the Moon around the Earth (a month); and the revolution of the Earth around the Sun (a year). These phenomena are independent of each other, so there is no direct correlation. It takes approximately 24 hours for the Earth to fully rotate. The Moon’s revolution is about 29.5 Earth days. The Earth’s revolution is about 365.25 Earth days, which is about 12.4 lunar months.
We are accustomed to one day starting with daytime and ending with nighttime. In more technical cases, the day starts at midnight (12 a.m.) and ends at 11:59 p.m.
In the Jewish calendar, each day starts at sundown. This is why when you look at some calendars it says “[insert Jewish holiday here] begins at sundown.” To be more specific, the Jewish day begins when the first three stars in the sky appear. It’s still essentially at sunset, but this is the exact way it’s done.
Like the Gregorian calendar, which most of us are familiar with, the Jewish calendar has one week made up of seven days. This lines up with what the Tanakh/Old Testament says.
|1||יום ראשון||Yom Rishon||First Day (Sunday)|
|2||יום שני||Yom Sheini||Second Day (Monday)|
|3||יום שלישי||Yom Shlishi||Third Day (Tuesday)|
|4||יום רביעי||Yom R’vi’i||Fourth Day (Wednesday)|
|5||יום חמישי||Yom Chamishi||Fifth Day (Thursday)|
|6||יום ששי||Yom Shishi||Sixth Day (Friday)|
|7||יום שבת||Yom Shabbat||Sabbath (Saturday)|
The Gregorian calendar, which most of us are familiar with, is a solar calendar. This is why the months don’t really correlate well with the moon cycles. So, the months are anywhere from 28 to 31 days long. The Jewish calendar is a lunar calendar, in which the months follow the Moon’s cycle and the Earth’s revolution around the sun. The months vary from 29 to 30 days.
In the Gregorian calendar, each year is 12 months long. In the Jewish calendar, each year is 12 months long. The exception to this is when it’s a leap year (a thirteenth month is added).
Every ecclesiastical new year falls on Nisan 1. This is as ordained in Exodus 12:2 and Deuteronomy 16:1. This set Nisan (previously Aviv) as “the first of months.”
|8||מַרְחֶשְׁוָן||Marcheshvan or Cheshvan||October-November|
|12||אֲדָר א׳||Adar I (I is added for leap years)||February-March|
|13||אֲדָר ב׳||Adar II||February-March|
The number of days vary every year because of the lunar months. We do know that each year is twelve months long, except leap years, which are 13 months long. Leap years come around every two or three years.
The year on the Jewish calendar represents the number of years since creation. This was calculated by adding up the ages of people in the Bible back to the time of creation. However, this does not mean that the universe has existed for only 5,700 years as we understand it. “Many Orthodox Jews will readily acknowledge that the first six ‘days’ of creation are not necessarily 24-hour days (indeed, a 24-hour day would be meaningless until the creation of the sun on the fourth ‘day’).” (Rich).
Now that we know this, we can determine that today’s date (12 August 2012) is 24 Av 5772.
Rich, Tracey R. “Judaism 101: Jewish Calendar.” Judaism 101: Jewish Calendar. N.p., n.d. Web. 12 Aug. 2012. .