Rabbi Hillel II developed the modern Hebrew/Jewish calendar in the Jewish year of 4119, or AD 359 on the Gregorian calendar. Using his calendar methods (as described in Part 2) and assuming that the Gregorian calendar we use today was in effect at that time, the dates of Rosh Hashanah would have ranged from August 29 to September 28, between the Hebrew years 4100 and 4200. In the present Jewish calendar, the 58th century, the dates of Rosh Hashanah range from September 5 to October 5, a gain of six or seven days. This slight difference of approximately one hundred minutes over nineteen years has accumulated to almost a week over a 1,650 year period. It takes almost 9,300 years for this discrepancy to accumulate to a full month in time.
Here is viewpoint on the Hebrew calendar as it corresponds to the agricultural season:
1st month on sacred calendar/7th month on civil calendar–Nisan Barley Harvest
2nd month on sacred calendar/8th month on civil calendar–Iyar Barley Harvest
3rd month on sacred calendar/9th month on civil calendar–Sivan Wheat Harvest
4th month on sacred calendar/10th month on civil calendar–Tamuz Grape Harvest
5th month on sacred calendar/11th month on civil calendar–Av Olive Harvest
6th month on sacred calendar/12th month on civil calendar–Elul Dates/Figs Harvest
7th month on sacred calendar/1st month on civil calendar–Tishri Early Rains
8th month on sacred calendar/2nd month on civil calendar–Heshvan Plowing
9th month on sacred calendar/3rd month on civil calendar–Kislev Wheat/Barley Sowing
10th month on sacred calendar/4th month on civil calendar–Tevet Winter Rains
11th month on sacred calendar/5th month on civil calendar–Shevat Almond Bloom
12th month on sacred calendar/6th month on civil calendar–Adar Citrus Harvest
13th month on sacred calendar–Intercalary Month
The celebrations of Passover, the Feast of Firstfruits and the Days of Unleavened Bread all occur in Nisan – during the barley harvest.
Pentecost occurs in Sivan, the time of the wheat harvest.
The last three grand Jewish celebrations: the Feast of Trumpets, the Day of Atonement and the Feast of Tabernacles all occur in Tishri, at the end of the major harvest.
A Couple of Last Points
There are two last peculiarities to the Hebrew calendar. Yom Kippur (the Day of Atonement) is the most holy day on the Hebrew calendar. This day should not fall adjacent to the seventh-day Sabbath because Atonement is a day when fasting, doing without food or water, occurs. This would cause difficulties in coordination of the weekly Sabbath.
The seventh day of the Feast of Tabernacles should not fall on a Saturday because that would interfere with the holiday's observances. A day is added to the month of Heshvan or subtracted from the month of Kislev of the previous year to prevent these events from happening. This process is sometimes referred to as "fixing" Rosh Hashanah.
There we have it. If you have read this three part series on the Hebrew calendar I'm more than confident that you now understand it very well. Now when you visit Israel you will have a much better idea of coordinating dates,