And God said, "Let there be lights in the expanse of the sky to separate the day from the night, and let them serve as signs to mark seasons and days and years" Genesis 1:14
The Jewish holidays appear to shift around relative to our, Western - Gregorian, calendar. Thus Tabernacles, for instance, may fall in September or October. This is because the Jewish calendar is Lunar while the western calendar is Solar. (See also Christian Calendar)
Every four years the western calendar has to add a day ( 29th February ) to keep the calendar in step with the Earth's progress around the Sun.
The Jewish calendar is a lunar calendar, with twelve lunar months of 29 or thirty days, which is about ten days short of a solar year, so seven years in every nineteen have an extra month. This ensures that the seasonal feasts keep to their correct season.
Rosh Chodesh is the New Moon, which is the start of each month. Biblically, Rosh Chodesh was determined by witnesses observing the first thin crescent of the moon. In days before modern communication the new moon could not be notified to all dispersed communities so Rosh Chodesh is celebrated on two consecutive days outside Israel.
In Genesis Ch1 v14 GOD said,
"Let there be lights in the expanse of the sky ..... to serve to mark the seasons and days of years."
Biblically, the dates can not be predetermined, but are fixed by observation.
See also Rosh Chodesh to understand the importance of observation of the New Moon, as opposed to calculation.
The same is true of the start of the year - The Month of the Aviv - The biblical year begins when the first new moon after the barley in the land of Israel reaches the state in its development that the Bible calls Aviv. Only by checking the state of the barley crop can we fulfill the Biblical commandment to "Keep the Month of the Aviv" (Deut 16:1). This obviously determines when the other feasts should be celebrated, for example, the Feast of Unleavened Bread; ( "at the time of the month of the Aviv, because in the month of the Aviv you went out of Egypt." - Exodus 34:18) (The 19 year cycle is an un-Biblical substitute for this observational method)
In Leviticus 23, Moses instructed the people that Nisan ( or Aviv or Abib ) was to be the first month. This is the religious calendar, as opposed to the civil calendar which starts at Rosh Hashanna.
The seven seasonal feasts start with Passover (Pessach) (14th Nisan), which celebrated the creation of God's nation of Israel, and followed through to Tabernacles (Sukot) which celebrates Ingathering ( both as a harvest celebration and looking to God's ingathering of the righteous).
Rosh Hashanna ( New Year ) ( described with Yom Kippur) is now celebrated on the day of the feast of Trumpets. Thus the Jews start the year with a call to self examination and repentance before God.
|Jewish calendar month||Corresponding western calendar months in 2000|
|Adar Bet (Leap month)||March April|
Feasts in the year 2013 to 2016 CE (western) (5774 to 5776 Jewish)
|Feast||Hebrew date||2015 - 2016||2016 - 2017||2017-2018|
|Rosh Hashanna||1 Tishri||3 Oct||21 Sept|
|Fast of Gedaliah||3 Tishri||5 Oct||24 Sept|
|Yom Kippur||10 Tishri||12 Oct||30 Sept|
|Sukot||15 Tishri||17 Oct||5 Oct|
|Shemini Atzeret (8th day)||22 Tishrei||24 Oct||12 Oct|
|Simchat Torah||23 Tishri||24 Oct||12 Oct|
|Chanukah||25 Kislev-3 Tevet||25 Dec *||13 Dec|
|Tu B'Shvat||15 Shvat||25 Jan 2016||11 Feb||31 Dec|
|Esther / Purim||14 -15 Adar||24 Mar||12 Mar||1 Mar|
|Erev Pessach (Seder Night)||after 14 Nisan||22 Apr||10 Apr||30 Mar|
|Pessach||15-22 Nisan||23 Apr||11 Apr||31 Mar|
|Counting of the Omer (1st day)||23 Nissan||24 Apr||12 Apr||1 Apr|
|Yom Hashoah||2 Iyar||5 May||1 May||18 Apr|
|Independence||3 Iyar||12 May||2 May||19 Apr|
|Lag B'Omer||Iyar 18||26 May||14 May||3 May|
|Yom Yerushalayim||27 Iyar||5 Jun||24 May||13 May|
|Sha Vu Ot||6 Sivan||12 Jun||31 May||20 May|
|17th of Tammuz||17 Tamuz||24 Jul||11 Jul||1 Jul|
|Tisha Be Av||9 Av||14 Aug||1 Aug||22 Jul|
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Incidentally, the Jewish calendar is calculated from Creation and therefore needs no BC or AD. Jewish reference to the Western, Gregorian, calendar are CE or BCE; CE standing for Common Era and Before Common Era.
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