And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also (Gen 1:16). The two lights were later called the sun and the moon (Gen. 37:9).
Here we introduce our great calendar/clock presupposition: The motions of both sun and the moon together (greater and lesser lights) must be used when building a Biblical calendar. No other method of marking time outside of sun and moon together will qualify as Biblical.
Further, Biblical years always begin in Spring, on the first day of the month called Abib, which means green ears. This month [Abib, or green ears] shall be unto you the beginning of months: it shall be the first month of the year to you (Exo. 12:2). Since this day comes around only once in every four seasons, this day will always fall near what is called the Spring or Vernal Equinox, the day when both the day period and the night period are equal, when there are most precisely twelve hours in the day (John 11:9) between sunup and sundown. There are other astronomical measures of the year, which vary slightly from the usual solar year, which can be discovered in books on astronomy. Suffice it to say, that a Biblical year is the precise time between Abib 1 and the following Abib 1, spring to spring.
There you have the basis for a Biblical Calendar, designed for the specific purpose of connecting God with his people, through signs. . . seasons. . . days and years, marked by the motions of Gods heavenly calendar/clock, the sun and the moon.
Even so, we can still build a Biblical calendar on the perfect, inerrant and complete word of God. A few brief verses give us plenty of foundation material. We need a definite, undisputed beginning point, where sun and moon begin the year in sync, and we need precise divider points, which mark individual months.
Our beginning point is established precisely, concerning the Passover season:
And the LORD spake unto Moses and Aaron in the land of Egypt saying, This month shall be unto you the beginning of months: it shall be the first month of the year to you (Exo 12:2). On the secular Gregorian calendar, the first month is January, and the secular year begins January first, without reference to the phase of the moon. On the Biblical calendar, the first month is Abib and the Biblical year begins on Abib 1, which is marked and defined in the heavens by the appearance of a new moon. On a clear day, such a new moon can sometimes be viewed right after sunset on the western horizon. About two weeks later, on the full moon of Abib 15, the Feast of Unleavened Bread begins. Thereafter, throughout the year, each month is marked by the cycle of the moon as viewed from earth.
We are told: Observe the month of Abib, and keep the passover unto the LORD thy God (Deut. 16:1). Even when we fail to visually sight the barely visible new moon, we can still observe the month of Abib. How is this done? Simply put, to observe is to watch the moon for at least a few minutes each night. After a few nights of such observation, one learns that both the phase and the position of the moon at sunset changes each night, in a precise procession. The moon grows, or waxes and moves across the sky, from west to east, until full, when it is observed rising in the east precisely as the sun is setting in the west. The cycle takes exactly half a month, or about two weeks. Then the moon begins to lessen, or wane, for another half month, which is followed by another new moon, and the cycle repeats. The night of the full moon is the undisputed middle of the month, assigned day fifteen. Two festivals (Unleavened Bread and Tabernacles) begin on the fifteenth day which is the full moon. These two important months become our pattern for the rest of the year. By assigning the full moon to day fifteen, and numbering forwards and backwards we get months of either 29 or 30 days in every case.
Once a person has observed the month of Abib, noting especially the Festival that begins on the fifteenth (Lev. 23:6), or full moon, he has a good idea of what to expect during the months following. A careful observer can know in advance the precise time and position of the next full moon and new moon, which occur precisely every 29.53059 days, or one lunar cycle.
Twelve lunar cycles of 29.53059 days each equals only about 354 days, about 11 days short of a solar year. To harmonize both sun and moon, and keep Passover in the month of Abib, meaning green ears (Exod. 9:31), it becomes necessary occasionally to intercalate a thirteenth month, usually done twice in each seven year cycle. Otherwise, Passover would fall before green ears, which always come close to the Spring or Vernal Equinox. The principle is not at all complicated. Someone must administer a calendar, whereby observers check the local crops, to see if green ears are available. If not, a thirteenth month is added to the old year, and the green ears of Abib come one month later.
The Bible rule is given quite simply. Barley must be in the green ear at the reference latitude of Palestine before Abib can begin. If there are no green ears, Abib comes one month later. God, not fallible man, is in full control of both the weather and a Biblical calendar. The question is really one of authority and submission, not one of substance. Knowledge of a Biblical calendar has been largely lost because Israelites have refused to submit to the authority of Gods word, and prove it through obedience in practice. It takes a few cycles of obedience to the knowledge we have, then God may choose to give more from His bountiful word. Obedience to the word of God is the key to keeping a Biblical calendar. Without obedience to His word, such a calendar becomes little more than a theological head trip ending in argument and estranged friendships.
One additional factor must be mentioned. God preserves his Word in history through the church of the living God, the pillar and ground of the truth (1Tim. 3:15). No one generation of Christians can fly solo and rediscover everything. For this reason, we should look at the historic calendar of the church. When we take such a look, we find that the calendar used in the Roman world generally was the calendar of Julius Caesar, dating from 46 BC. This calendar is identical with the updated Gregorian calendar of the Christian world today, except that ten calendar days were dropped from one month under Pope Gregory in 1582 AD. Now the Julian/Gregorian calendar makes no claim as having a Biblical basis.
The Biblical calendar of Christs day, used by Israelites who had preserved it from antiquity, fell into disuse when Jerusalem was overrun in 70 AD. Christian Israelites continued to keep Passover, prior to the change in favor of pagan Easter, under Emperor Constantine of the fourth century. The church at Rome had changed to pagan Easter much earlier. Christians in the British Isles continued to keep Passover by the ancient rule, which indicates that Passover was kept on the first full moon following the spring equinox. Whatever rule is used requires administration, which God has given to His living church, which is given the mandate to Prove all things; hold fast that which is good (1Ths. 5:21).
Godly individuals will support those assigned administration of the calendar, communicate with them (Gal. 6:6), and pray for their wise decisions. Where church leaders are committed to following Bible truth, they need every help and support they can get. And, finally, a Biblical calendar is as important under the New Covenant as it was under the Old. The apostle Paul kept Pentecost (Acts 20:16; 1Cor. 16:8), and the Day of Atonement (Acts 27:9), among other Biblical holydays. He said: Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days (Col. 2:16). Though this verse is commonly used against Bible sabbaths, holydays and other events, it singularly specifies that special Biblical calendar days have respect, regardless of unbelief by nay sayers who refuse them.
To simplify, and summarize, a Biblical calendar is revealed by God in His Word, observed and obeyed by man. If most of the world blew up tomorrow, and all civilization as we know it were destroyed, and if you found yourself isolated as a survivor you would be able to immediately get in harmony with the new moons and full moons, by visual observation every night. With the aid of your Bible alone, you would then be able to coordinate the first new moon and full moon after the first barley plants came into green ears, and thereby know quite accurately when to observe Abib 1 and Passover (Abib 14), and the Days of Unleavened Bread, (Abib 15-21).
You would then count 50, and know precisely when to celebrate Pentecost on a Sunday. By observing the phases of the moon all summer, and writing down only a few references, you could know exactly when the seventh month, in the first day of the month, (Lev. 23:24) would come to the area where you would Blow up the trumpet in the new moon (Psa 81:3), and celebrate the Feast of Trumpets. Ten days later you would celebrate Atonement, and in the time appointed, [Heb. kece´=full moon] on our solemn feast day (v. 3), commonly called the harvest full moon, you would celebrate the Feast of Tabernacles, followed by the Last Great Day.
By continuing to observe the sun and moon throughout the winter, and by watching the early growth of barley, you could identify the month of Abib when the green ears burst forth. The ancient church rule of following the first full moon after the Spring Equinox for dating Passover is nearly always in harmony with the appearance of the green ears of Abib in Palestine.
Thus we learn a great lesson. The calendar/clock that God has given us in the Bible is to mark time for a great purpose. Through His Feast days, He reveals Himself and His divine plan. Without obedience in practice, however, a Biblical calendar falls flat to no purpose, as mere words and numbers on paper. Obedience of truth brings more truth, and Godly men build generation upon generation. He meets with His people through his Divine appointments called Feast days. It is important to know precisely when those days fall on the calendar, or we miss divine appointments. He teaches us, when we yield to his Word in obedience.
Praise God for a Biblical calendar, revealed in His Word.