Jewish Calendar

Jewish communities around the world use the Jewish or Hebrew calendar to determine the dates of religious observances and rituals. 
 
A Luni-solar calendar, the ‘normal’ calendar consists of 12 months of 29 or 30 days however, because it needs to be adjusted so that its months coincide with seasonal months, a leap year adds an extra month.

The basis of the Jewish annual calendar is ancient. With the Torah speaking of the annual cycle of Jewish Festivals, the sages systematised our calendar well before the destruction of the Second Temple in 70CE to enable us to celebrate and commemorate our Holidays and Festivals in sync with the agricultural seasons in which they originally occurred. 

For a list of upcoming Jewish Holidays, click here.

Days & Weeks:
The Jewish day begins and ends at sunset. Why? Because, in the biblical account of the Creation story in Genesis/Bereshit 1, we are told: there was evening, and there was morning—the first day; … there was evening, and there was morning—the second day; … there was evening, and there was morning—the third day; etc.

With the exception of Shabbat, the weekdays have no names but are, rather, simply numbered:

1. יום ראשון= yom rishon = first day = Sunday
2. יום שני=  yom sheni= second day = Monday
3. יום שלישי = yom sh’lishi = third day = Tuesday
4. יום רביעי= yom revi’i = fourth day = Wednesday
5. יום חמישי= yom chamishi = fifth day = Thursday
6. יום שישי= yom shishi= sixth day = Friday

Because the week culminates in the seventh day, Shabbat, Saturday, שבת, we Jews count weeks from Shabbat to Shabbat.

Months:
The Jewish month is based on the lunar month: that is, the time it takes for the moon to circle the earth.  Since the exact duration of one revolution is a little over 29.5 days, the length of the months normally alternates between 29 and 30 days.  

A month begins with the appearance of the new moon.  When the Temple stood, the Sanhedrin [the highest court] sanctified the new month upon the testimony of two reliable witnesses had actually sighted the moon. 

The current Jewish fixed calendar was introduced in the middle of the fourth century CE.

In the Torah, the months are numbered with Nisan, the first, being the one during tradition understands that the Exodus from Egypt took place. Later, names of Babylonian origin were adopted:

1. ניסן= Nisan= 30 days
2. אייר= Iyyar= 29 days
3. סיון= Sivan= 30 days
4. תמוז= Tammuz= 29 days
5. אב= Av= 30 days
6. אלול= Elul= 29 days
7. תשרי=Tishri= 30 days
8. חשון= Cheshvan= 29 or 30 days
9. כסלו= Kislev= 30 or 29 days
10. טבת= Tevet= 29 days
11. שבט= Sh’vat= 30 days
12. אדר= Adar= 29 days

Years:
Ancient peoples counted their calendar years anew with the reign of each new monarch. When Christianity rose to dominance, it dated history from the birth of their ‘king’, a practice reflected in our everyday, Gregorian calendar.

But Judaism could not consent to divide history along this lines and nor could we agree to divide universal history using the birth of even Abraham or Moses as a fulcrum. For many centuries, then, we Jews counted historical years from the key foundational event in our history: the Exodus from Egypt. The destruction of the Temple in 70CE was so cataclysmic that, for a while, it replaced the use of the Exodus as the inaugural date for counting time.

Centuries later, it became clear that even an event of this magnitude was not critical enough to draw a line through time and to restart counting world history. Only one occurrence could possibly serve as a beginning for history and so Judaism determined to count the years of the calendar on a universal scale from the creation of the universe.

The problem, however, remains: exactly how old is the earth?
Tradition and science vary, vastly, in quantum – but then tradition/religion is not science and science is not tradition/religion.