Special Note: These are reference sources only – God's Word is held as the only source of truth. Accordingly,
where the references differ from the original Bible text they are considered to be in error.
The major reference sources appear to be very sketchy on wheat harvesting in July-August, but God clearly states in Deut 16:13 that wheat was being harvested shortly before this time: “…‘You shall observe the Feast of Tabernacles seven days, when you have gathered from your threshing floor and from your winepress.’…” [NKJV]. The early harvesting of wheat was done in May and shortly thereafter the festival of firstfruits was celebrated (Pentecost), but the major part of the harvest (the late wheat harvest) was not completed until after Pentecost, thus extending to the harvest season of grapes and other fruits – this is known by God's reference to both the "threshingfloor" and "winepress" in Deut 16:13. Also, in Lev 26:3-5 it is stated, “…'If you follow my decrees and are careful to obey my commands, 4 I will send you rain in its season, and the ground will yield its crops and the trees of the field their fruit. 5 Your threshing will continue until grape harvest and the grape harvest will continue until planting, and you will eat all the food you want and live in safety in your land.’…” [NIV].
The grain harvests were comprised of three major elements, being reaping, threshing and winnowing, all of which were done in quick succession in the grain field itself. Accordingly, where one of these three elements is mentioned we can know that the reference is to harvesting as a whole. There are many studies on the matter that support this position, but to name just a couple for quick reference purposes there is An Ethnoarchaeological Study of Prehistoric Agriculture in the Ethiopian Highlands  and a succinct paper on Canaan and Ancient Israel .
Originally the Israelites were obedient to God, therefore we can know with certainty that after initial entry into the land of Canaan the wheat harvest continued until well after Pentecost and from Scripture it appears to have extended past Pentecost for about three (3) months, or about mid-late August. The importance of these Scriptures cannot be emphasized enough, because they show the seasons as they should be, as intended by God, not as they appear now or even 2,000-3,000 years ago. The period of obedience relating to Lev 26:5 and mentioned more specifically in Judges 2:7 continued for about fifty (50) years according to Barnes' commentary on verse 7. Because of Israel's disobedience after that time God lifted His protection of the nation and from this it is reasonable to conclude that the seasons also changed, the rains did not fall in due season and the land did not yield its abundance. As a result, a large body of commentary relating to the Biblical harvest seasons is at best misleading. Only God's Word can be relied upon to give an accurate account of the seasons, as God intended them to be understood and relate correctly to His annual Holy Days.
An important point to bear in mind is that the wheat harvest is actually a single harvest depicting the harvesting or ascension of humanity into the family of God. With the ideal conditions under the Old Covenant (which Millennial-like conditions only lasted for about 50 years) it appears the wheat harvest season actually spanned a period of four (4) months. Pentecost is the festival to celebrate the first part, or beginning, of the harvest and the Last Great Day is the festival to celebrate the completion of the remaining part of the harvest. As the remainder, or "second" harvest, it is always going to be much larger than the "first" harvest, which only comprises the beginning part of the overall harvest.
Ethnoarchaeological Approaches to the Study of Prehistoric Agriculture in the Ethiopian Highlands by: A.C. D'Andrea D.E. Lyons Department of Archaeology, Simon Fraser University, Burnaby, British Columbia, Canada. Mitiku Haile Mekelle University College, Mekelle, Ethiopia. E.A. Butler Institute of Archaeology, University College, London, UK. This paper can be found in The Exploitation of Plant Resources in Ancient Africa, edited by Van der Veen. Kluwer Academic: Plenum Publishers, N ...
one month of barley harvesting. one month of harvesting and measuring (wheat) two months of cutting ... on threshing ... ANCIENT AND MODERN
The Hebrew calendar (Hebrew: הלוח העברי) or Jewish calendar is the annual calendar used in Judaism. It determines the dates of the Jewish holidays, the appropriate Torah portions for public reading, Yahrzeits (the date to commemorate the death of a relative), and the specific daily Psalms which some customarily read. Two major forms of the calendar have been used: an observational form used prior to the destruction of the Second Temple in 70 CE, and based on witnesses observing the phase of the moon, and a rule-based form first fully described by Maimonides in 1178 CE, which was adopted over a transition period between 70 and 1178.
The "modern" form is a rule-based lunisolar calendar, akin to the Chinese calendar, measuring months defined in lunar cycles as well as years measured in solar cycles, and distinct from the purely lunar Islamic calendar and the almost entirely solar Gregorian calendar. Because of the roughly 11 day difference between twelve lunar months and one solar year, the calendar repeats in a Metonic 19-year cycle of 235 lunar months, with an extra lunar month added once every two or three years, for a total of seven times every nineteen years. As the Hebrew calendar was developed in the region east of the Mediterranean Sea, references to seasons reflect the times and climate of the Northern Hemisphere.
Jews have been using a lunisolar calendar since Biblical times. The first commandment the Jewish People received as a nation was the commandment to determine the New Moon. The beginning of Exodus Chapter 12 says "This month (Nissan) is for you the first of months.". The months were originally referred to in the Bible by number rather than name. Only four pre-exilic month names appear in the Tanakh (the Hebrew Bible): Aviv (first; literally "Spring", but originally probably meant the ripening of barley), Ziv (second; literally "Light"), Ethanim (seventh; literally "Strong" in plural, perhaps referring to strong rains), and Bul (eighth), and all are Canaanite names, and at least two are Phoenician (Northern Canaanite). It is possible that all of the months were initially identifiable by native Jewish numbers or foreign Canaanite/Phoenician names, but other names do not appear in the Bible.
Furthermore, because solar years cannot be divided evenly into lunar months, an extra embolismic or intercalary month must be added to prevent the starting date of the lunar cycles from "drifting" away from the Spring, although there is no direct mention of this in the Bible. There are hints, however, that the first month (today's Nissan) had always started only following the ripening of barley; according to some traditions, in case the barley had not ripened yet, a second last month would have been added. Only much later was a systematic method for adding a second last month, today's Adar I, adopted.
During the Babylonian exile, immediately after 586 BCE, Jews adopted Babylonian names for the months, and some sects, such as the Essenes, used a solar calendar during the last two centuries BCE. The Babylonian calendar was the direct descendant of the Sumerian calendar.
Names and lengths of the months
During leap years Adar I (or Adar Aleph — "first Adar") is considered to be the extra month, and has 30 days. Adar II (or Adar Bet — "second Adar") is the "real" Adar, and has 29 days as usual. For example, in a leap year, the holiday of Purim is in Adar II, not Adar I.
When does the year begin?
According to the Mishnah (Rosh Hashanah 1:1), there are four days which mark the beginning of the year, for different purposes:
There may be an echo here of a controversy in the Talmud about whether the world was created in Tishri or Nisan; it was decided that the answer is Tishri.
The epoch of the modern Hebrew calendar is 1 Tishri AM 1 (AM = anno mundi = in the year of the world), which in the proleptic Julian calendar is Monday, October 7, 3761 BCE, the equivalent tabular date (same daylight period). This date is about one year before the traditional Jewish date of Creation on 25 Elul AM 1. (A minority opinion places Creation on 25 Adar AM 1, six months earlier, or six months after the modern epoch.) Thus, adding 3760 to any Julian/Gregorian year number after 1 CE will yield the Hebrew year which roughly coincides with that English year, ending that autumn. (Add 3761 for the year beginning in autumn). Due to the slow drift of the modern Jewish calendar relative to the Gregorian calendar, this will be true for about another 20,000 years.
The traditional Hebrew date for the destruction of the First Temple (3338 AM) differs from the modern scientific date, which is usually expressed using the Gregorian calendar (586 BCE). The scientific date takes into account evidence from the ancient Babylonian calendar and its astronomical observations. In this and related cases, a difference between the traditional Hebrew year and a scientific date in a Gregorian year results from a disagreement about when the event happened — and not simply a difference between the Hebrew and Gregorian calendars. See the "Missing Years" in the Hebrew Calendar.
Measurement of the month
The Hebrew month is tied to an excellent measurement of the average time taken by the Moon to cycle from lunar conjunction to lunar conjunction. Twelve lunar months are about 354 days while the solar year is about 365 days so an extra lunar month is added every two or three years in accordance with a 19-year cycle of 235 lunar months (12 regular months every year plus 7 extra or embolismic months every 19 years). The average Hebrew year length is about 365.2468 days, about 7 minutes longer than the average tropical solar year which is about 365.2422 days. Approximately every 216 years, those minutes add up so that the modern fixed year is "slower" than the average solar year by a full day. Because the average Gregorian year is 365.2425 days, the average Hebrew year is slower by a day every 231 Gregorian years. During the last century a number of Jewish scholars suggested that the chief rabbinate in Jerusalem consider modifying this rule to avoid this effect.
Measurement of "molads" (lunar conjunctions)
The calendar is based on mean lunar conjunctions called "molads" spaced precisely 29 days, 12 hours, and 793 parts apart. Actual conjunctions vary from the molads by up to 7 hours in each direction due to the nonuniform velocity of the moon. This value for the interval between molads (the mean synodic month) was measured by Babylonians before 300 BCE and was adopted by the Greek astronomer Hipparchus and the Alexandrian astronomer Ptolemy. Its remarkable accuracy was achieved using records of lunar eclipses from the eighth to fifth centuries BCE. Measured on a strictly uniform time scale, such as that provided by an atomic clock, the mean synodic month is becoming gradually longer, but since the rotation of the earth is slowing even more the mean synodic month is becoming gradually shorter in terms of the day-night cycle. The value 29-12-793 was almost exactly correct at the time of Hillell II and is now about 0.6 s per month too great. However it is still the most correct value possible as long as only whole numbers of parts are used. Especially, it is far more accurate than the average solar year due to the 19-years-235-months equality described above — the total accumulated error of 29-12-793 from its Babylonian measurement until the present amounts to only about five hours.
The average length of the month assumed by the calendar is correct within a fraction of a second (although individual months may be a few hours longer or shorter than average). There will thus be no significant errors from this source for a very long time. However, the assumption that 19 tropical years exactly equal 235 months is wrong, so the average length of a 19 year cycle is too long (compared with 19 tropical years) by about 0.088 days or just over 2 hours. Thus on average the calendar gets further out of step with the tropical year by roughly one day in 216 years. If the intention of the calendar is that Pesach should fall on the first full moon after the vernal equinox, this is still the case in most years. However, at present three times in 19 years Pesach is a month late by this criterion (as in 2005). Clearly, this problem will get worse over time and if the calendar is not amended, Pesach and the other festivals will progress through a complete cycle of seasons in about 79,000 years.
As the 19 year cycle (and indeed all aspects of the calendar) is part of codified Jewish law, it would only be possible to amend it if a Sanhedrin could be convened. It is traditionally assumed that this will take place upon the coming of the Messiah, which will mark the beginning of the era of redemption according to Jewish belief. Theoretically, if Jewish law could be modified, one solution would be to replace the 19-year cycle with a 334-year cycle of 4131 lunations. This cycle has an error of only one day in about 11,500 years. However, this would be impossibly cumbersome in practice. Further, no such mathematically fixed rule could be valid in perpetuity, because the lengths of both the month and tropical year are slowly changing. Another possibility would be to calculate the approximate time of the vernal equinox and have a leap year if and only if Pesach would otherwise start before the vernal equinox. Similar ideas are used in the Chinese calendar and some Indian calendars.