The Western calendars are based on the Gregorian solar calendar of 365 days (Wikipedia Gregorian calendar).
Othes countries use the lunar calendar. The lunar year has between 354 and 355 days. There is therefore a difference of about 11 days between the solar calendar (365 days) and the lunar calendar (354 days). Nevertheless, the Hebrew lunar calendar, also takes into account the solar cycle of 365 days, so it is a "lunisolar" calendar (Wikipedia Hebrew lunar calendar).
The important points to know about the biblical calendar, like the Hebrew calendar, are the following (important to understand the biblical reasoning that will follow):
In the Bible, the days begin in the evening after the sunset. For example in Genesis 1:5, regarding the first day of the symbolic “And there was evening and there was morning, a first day".
In the Bible, the months began at the "new moon" (Psalms 81:3). Lunar months are either 30 or 29 days.

Nevertheless, at this stage of the study, there is a significant difficulty regarding the biblical definition of the new moon (Wikipedia New Moon). Some think that this is the moment when the moon completely disappears (in the current Hebrew calendar). Others think that this is the first crescent of the moon. For this reason, there is a special study of the theme of the New Moon that will indicate the necessary and important choice between these two options to determine exactly a biblical date (that of the memorial to the death of Jesus Christ (Nisan 14th) and Tishri 10th) (After this article (below)).


The lunar year has 12 months of 29 or 30 days. The intercalary month (Veadar) is regularly added to compensate for the difference of 11 days between the lunar year (354 days) and the solar year (365 days). In this case, this year has exceptionally 13 months, it is a “lunisolar”. In the Bible there is no direct mention of this process in order to adjust the lunisolar calendar to the solar year. Nevertheless, indirect information shows that this adjustment was made. For example, some months had to coincide regularly with seasonal crops, either spring (Abib (Nisan): spring) or autumn (Ethanim (Tishri): autumn) (Exodus 23:15). It is obvious that the Israelites of the biblical times realized this adjustment by the addition of the intercalary month (Veadar). The festivals celebrated the month in spring, remained spring, over the years. And all the autumn festivals of the month of Tishri, remained autumnal, over the years (Leviticus 23:37).

Currently the adjustment system, used by the Hebrew calendar, is made with a “metonic cycle”: 7 adjustments with 7 lunisolar years of 13 months, over a period of 19 years.

In the book of Ezekiel we can understand that the year in which the Great Tribulation will occur will be Lunisolar, with 13 months, according to the Jewish calendar. In Ezekiel chapters 38 and 39 we have the prophetic story of events before, during and after the Great Tribulation. After the Great Tribulation the prophecy gives the information of a seven-month period of cleansing the Earth: "The house of Israel will spend seven months burying them in order to cleanse the land" (Ezek. 39:12-14). How would this simple information make us understand that this would be a year of 13 months?

According to Revelation 11:19 the Great Tribulation will take place a Tishri 10th. At the end of the seven months written in Ezekiel 39: 12-14, it is written that the prophet had the vision of the Temple which represents the dominion of the Kingdom of God on earth, a certain in Nisan 10th: " In the 25th year of our exile, at the beginning of the year, on the tenth day of the month, in the 14th year after the city had fallen, on that very day the hand of Jehovah was upon me, and he took me to the city" (Ezekiel 40:1)

The simple cross-checking of information from the 7 months period, with the period from Tishri 10th to Nisan 10th (6 months normally), we understand that there will be the intercalary month of Veadar: The year when the Great Tribulation will take place will be a Lunisolar Year of 13 months.

New Moon

"Blow the horn on the new moon, On the full moon, for the day of our festival"

(Psalms 81:3)

There are two definitions of the new moon:

1 - "The new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the unaided eye, except when silhouetted during a solar eclipse. Daylight outshines the earthlight that dimly illuminates the dark side of the new Moon. The actual phase is usually a very thin crescent because the Moon rarely passes directly in front of the Sun, except in a solar eclipse" (Wikipedia New Moon).

2 - "The original meaning of the term new moon, which is still sometimes used in non-astronomical contexts, was the first visible crescent of the Moon, after conjunction with the Sun. This crescent Moon is briefly visible when low above the western horizon shortly after sunset and before moonset" (Wikipedia New Moon).

The important question is what is written in the Bible to help us to understand what the new moon is. Psalm 81:3 allows us to have a precise answer.

This text shows that the first day of the month, on the "new moon" was announced by the blowing the horn. And then, the Psalm informs that on the "full moon" was celebrated the day of a "festival". The context can allow us to know what were this festival and its date. With this information we can know what new moon it is.

Before that, based on the astronomical table of the lunar cycle of November 2018 (as example), we must know that the complete disappearance of the moon (without crescent), usually lasts three nights (see the illustration: the nights of 7,8,9). The two letters in blue "NL", is the moment when the moon emits the least residual light and consequently it is considered as New Moon (by astronomical measure). It is usually between the first and the second moonless night. Nevertheless, by putting ourselves in the place of humans who had no astronomical instrument of measurement, it is logical to think that, in the case where the new moon was at the time of the moonless night, the new moon was logically the first moonless night.

And the full moon, the fully enlightened moon (whatever the intensity of the sunlight), lasts three nights (22,23,24 (example)). Applying the same logic of a human without an astronomical measuring instrument, the full the moon was the first night when the moon appears fully, that is, in this example, on November 22nd.

The context of Psalm 81 suggests that was the festival of booths with a trumpet blast (or horn) at Tishri 1st: "Tell the Israelites, ‘In the seventh month, on the first of the month, you should observe a complete rest, a memorial signaled by a trumpet blast, a holy convention" (Leviticus 23:24). Then in Tishri 15th was the festival of booths (Leviticus 23:34). Regarding this festival Jehovah had ordained: "Seven days you will celebrate the festival to Jehovah your God in the place that Jehovah chooses, for Jehovah your God will bless all your produce and all that you do, and you will become nothing but joyful" (Deuteronomy 16:13-15 compare with Psalm 81:1,2 "Shout joyfully to God our strength. Shout in triumph to the God of Jacob").

If the 15th of the month represents the full moon (whether it is the first day or the second day (full moon)), going up we reach the first day, the new moon, the moonless night. This option (of the new moon (moonless night)) was retained by the current Hebrew calendar. And it is interesting to note that the Christians who calculated the date of October 4/5, 1914, as the date of the enthronement of Christ in heaven, Tishri 15th (4/5 October 4/5th), have also retained this new moon option (moonless night (not the first crescent)). By the way, the method of considering the new moon by the first visible crescent depends on the geographical location where one is. So the schedule could vary from one day depending on the geographical location where the observer would be ...

The option of the new moon, the moonless night, (and not that of the first crescent moon (which has no biblical basis)), conforms to the Bible. This option was chosen by the Hebrew calendar and it is on this basis that the date of the commemoration of the death of Christ (which is mainly based on the current Hebrew calendar) is calculated and the determination of the date of Tishri 10th.