We often Keep track of certain events with the aid of a calendar. Have you ever noticed that certain events keep shifting dates each year? Why are there different methods for calculating time? How do the different calendars interact with one another in modern times. Lets explore the origins in dating methods for festivals observed to this day.
Most of us set our schedule around what has become known as the Gregorian calendar. As you will see below the names we associate each day is derived from another type of calendar. The Gregorian calendar is a modification of the Julian calendar. However the Gregorian calendar is not perfect; leap years make up for the fact that a solar year is not exactly 365 days. Jewish events fall under the lunar cycle that can be traced to origins in Babylonia. Following the Babylonian conquest in 586 BCE, the exiled people of Judea arrived in Babylonia and confronted a sophisticated and very different society. There they absorbed many aspects of the general culture (such as calendrical names and the local language and writing system), but maintained their own separate religion and traditions.
The ancient Babylonian calendar was based on the moon. There was a holy day at each quarter in the moon’s cycle. This happens about every seven days. Seven became a mystical number- the seven days in a week were named after god and planets, associated with objects that moved through the sky. Sunday= the Sun’s day, Monday= the Moon’s day, Tuesday= Tiw’s ( a Norse name for Mars), Wednesday= Woden’s day (Norse name for Mercury), Thursday= Thor’s day (North name for Jupitar), Friday= Frigg’s day (Frigg is the Norse name for Venus), Saturday= Saturn’s day.
Although the basic structure of the Jewish calendar is based on the Babylonian system, the ancient Israelites’ recognition of the Sabbath set their calendar apart.The Hebrew calendar uses the days of the week as noted in Genesis 1:5-2:3 and named them based on the story of creation.
As with the Babylonian calendar, the monthly cycles of the Jewish calendar are dependent on the moon, and the names of the months are based upon the Babylonian model. The Babylonians kept track of the constellations which lead to the formation of the 12 signs of the Zodiac (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius,Pisces.. or corresponding 12 months of the Hebrew calendar- Aries= Nisan, Taurus=Iyar, Gemini=Sivan, Cancer=Tamuz, Leo=Av, Virgo=Elul, Libra=Tishrei, Scorpio=Cheshvan, Sagittarius= Kislev, Capricorn=Tevet, Aquarius= Shvat, Pisces=Adar. Proof to this can be found in the mosaic synagogue floor of Beth Alpha The Jewish calendar is lunisolar-months are based on the phases of the moon and years are based on the rotation of the earth around the sun. Occasionally this necessitates the addition of a thirteenth month to the twelve-month Jewish calendar.
Today’s Jewish calendar contains some of the festivals celebrated in ancient times, but many changes and additions have been made over the past several thousand years. The 9th of Av (Tisha b’Av) is a holiday commemorating the destruction of the First Temple, which occurred in 586 BCE and prompted the exile in Bablyon. The Hebcal online application allows you to see into the past and into the future all dates as they would fall in the Hebrew Calendar.
In Mishnah, section Rosh Hashanah 1 we are exposed to the codification of the calendar:
The Mishnah (Rabbinic law) Rosh Hashanah 1 we deal with the fixing of the calendar, specifying four New Year periods and prescribing the manner of observing the moon to determine the lunar months. The passage shows disputes among leading authorities:
1: 1 There are four “New Year” days: on the first of Nisan is the New Year for kings and feasts; on the first of Elul is the New Year for the Tithe of Cattle (Rabbi Eleazar and Rabbi Simeon say: The first of Tishre); on the first of Tishre is the New Year for [the reckoning of] the year [of foreign kings], of the Years of Release 173 and Jubilee years, for the planting [of trees] and for vegetables; 174 and the first of Shevat is the New Year for [fruit-]trees (so the School of Shammai, and the School of Hillel say: On the fifteenth thereof).

2 At four times in the year is the world judged: at Passover, regarding grain; at Pentecost, regarding the fruits of the tree; on New Year’s Day all that come into the world pass before Him like legions of soldiers, for it is written, “He that fashions the hearts of them all, that considers all their works” (Ps. 33:15); and at the Feast [of Tabernacles] they are judged regarding water.

3 Because of six New Moons do messengers go forth [to proclaim the time of their appearing]: because of Nisan, to determine the time of Passover, because of Av, to determine the time of the Fast; because of Elul, to determine the New Year; because of Tishre, to determine aright the set feasts; because of Kislev, to determine the time of[the feast of] the Dedication (Hanukkah); and because of Adar, to determine the time of Purim. And while the Temple still stood they went forth also because of Iyyar, to determine the time of the Second Passover. 175

4 Because of two New Moons may the Sabbath be profaned: 176 [the New Moon] of Nisan and [the New Moon] of Tishre, for on them messengers used to go forth to Syria, and by them the set feasts were determined. And while the Temple still stood the Sabbath might also be profaned because of any of the New Moons, to determine aright the time of the offerings.

5 Whether [the New Moon] was manifestly visible or not, they may profane the Sabbath because of it. Rabbi Yose says: If it was manifestly visible they may not profane the Sabbath because of it.

6 Once more than forty pairs [of witnesses] came forward, but Rabbi Akiva in Lydda restrained them. Rabban Gamliel sent to him [saying]: “If you restrain the multitude, you will put a stumbling-block in their way for the future.” 177

7 If a father and his son saw the New Moon they may [both] go [to bear witness]; not that they can be included together [as a valid pair of witnesses], but that if one of them is found ineligible, the other may be included to make a pair with some other [witness]. Rabbi Simeon says: A father and his son, and any that are near of kin, are eligible to bear witness about the New Moon. Rabbi Yose said: Once Toviah the Physician saw the New Moon in Jerusalem together with his son and his freed slave; and the priests accepted him and his son but pronounced his freed slave ineligible. And when they came before the court, they accepted him and his slave but declared his son ineligible.

8 These are they who are ineligible (as witnesses): a dice-player, a usurer, pigeon-flyers, traffickers in Seventh Year produce, and slaves. This is the general rule; any evidence that a woman is not eligible to bring, these are not eligible to bring.

9 If a man saw the New Moon but could not walk, he may be taken on an ass [on the Sabbath] or even on a bed; and if any lie in wait for them, they may take staves in their hands. If it was a far journey, they may take food in their hands, since for a journey enduring a night and a day they may profane the Sabbath and go forth to bear witness about the New Moon, for it is written, “These are the set feasts of the Lord, even holy convocations which you shall proclaim in their appointed season” (Lev. 23:4).

As we have seen the Hebrew calendar of today has biblical origins but freely borrowed from ancient cultures.