A system of reckoning time, usually based on a recurrent natural cycle (such as the sun through the seasons or the moon through its phases); a table, or tabular register, of days according to a system usually covering one year and referring the days of each month to the days of the week.
From the beginning of recorded history, the calendar has been used to keep records and predict the time for the changing of the seasons. The calendar provided a framework in which man could plan his work. It was an effective timetable for marking various religious festivals that were to be celebrated at regular intervals.
The day - In calendar terms, the day is the smallest and most consistent unit of time. In the ancient world, the term day was used in two senses. It described a 24-hour period, as well as daylight in contrast to the night (Gen 1:5). The beginning point of the 24-hour day varied. The Bible contains references to the day beginning in the morning (Gen 19:34; Acts 23:32) as well as in the evening (Neh 13:19). In the time of the Roman Empire, the day may have begun at midnight, as indicated by the Gospel of John (4:6; 19:14).
The dawn was the twilight before sunrise (1 Sam 30:17; Matt 28:1). The evening was the late afternoon (Deut 16:6) between the day and the night (Jer 6:4; Prov 7:9), or it could mean literally "late" in the day (Mark 11:19) just before the stars came out (Neh 4:21). Noon was the end of the morning (1 Kings 18:26) which marked mealtime (Gen 43:16). Noon was also referred to as "midday" (Neh 8:3), "broad daylight" (Amos 8:9), and "heat of the day" (2 Sam 4:5).
The day was divided into three parts: evening, morning, and noon (Ps 55:17). Midnight was the midpoint of the night (Matt 25:6; Acts 20:7). In the Old Testament the night was divided into three watches (Judg 7:19; Ex 14:24), while it was divided into four watches in the New Testament (Matt 14:25; Mark 13:35). The term hour was used to mean "immediately" (Dan 3:6,15), or it could express the idea of one-twelfth of daylight (John 11:9).
The week - The week was a seven-day unit begun at the time of creation (Gen 1:31-2:2). The word week means "seven" (Gen 29:27; Luke 18:12). In the Bible the days of the week were called the "first day," "third day," and so forth (Gen 1:8-31; Matt 28:1), although the seventh day was known as "sabbath" (Ex 16:23; Matt 12:1). The day before the Sabbath was called "the Preparation Day" (Mark 15:42), and Christians referred to the first day of the week as "the Lord's Day" (Rev 1:10).
The month - The month was a unit of time closely tied to the moon. The Hebrew word for "month" also meant "moon" (Deut 33:14, NIV, NASB). The reason for the connection between the month and the moon is that the beginning of a month was marked by a new moon. The moon was carefully observed by the people of Bible times. When it appeared as a thin crescent, it marked the beginning of a new month.
The lunar month was about 29 days long. Therefore, the first crescent of the new moon would appear 29 or 30 days after the previous new moon. At times the crescent was not visible because of clouds. But this was allowed for with a rule that the new moon would never be reckoned as more than 30 days after the last new moon. This prevented too much variation in the calendar.
The year - The Hebrew word for year comes from the idea of change or repeated action. Thus the year expresses the concept of "a complete cycle of change." Due to the repeated seasons, man set up a calendar to account for yearly events and to alert him of the coming seasons. The calendar revolved around the agricultural cycles. Man observed the climatic changes and the length of days in his planting and harvesting. Religious festivals were also established to parallel the agricultural year. No major religious festival, for example, was celebrated during the busy harvest season. Man observed that there were four seasons and that the year was about 365 days long. Although the calendars were not always precise, adjustments were made periodically to account for the lack of precision.
In the Old Testament - The marking of time in Old Testament days revolved primarily around the months, seasonal religious festivals, and the year. The month was marked by the first appearance of the crescent of the new moon at sunset. The first day of each month was considered a holy day marked by special sacrifices (Num 28:11-15), and it was to be announced with the blowing of trumpets (Num 10:10; Ps 81:3).
Normally the months were designated numerically: first (Ex 12:2), second (Ex 16:1), third (Ex 19:1), fourth (2 Kings 25:3), fifth (Jer 28:1), sixth (1 Chron 27:9), seventh (Gen 8:4), eighth (Zech 1:1), ninth (Ezra 10:9), tenth (Gen 8:5), eleventh (Deut 1:3) and twelfth (Est 3:7).
The first month of the Hebrew calendar was in the spring, around March/April. In their early history the Israelites adopted Canaanite names for the months which were connected with agriculture and climate. Only four of these names are mentioned in the Old Testament. The month Abib (Ex 13:4; 23:15) was the first month (around March/April), which was at the time of barley harvest. The word Abib means "ripening of grain" (Lev 2:14). The month Ziv (1 Kings 6:1,37; Zif, KJV) was the second month (April/May). This word means "splendor," and it refers to the beauty of flowers blooming at that time. Ethanim (1 Kings 8:2) was the seventh month (September/October), which occurred during the rainy season. Bul (1 Kings 6:38) was the eighth month (October/November). Its name may have reference to "rain," since the eighth month was between the early and latter rains. These four names for the months were associated with the most important agricultural times of the year.
In its later history the nation of Israel adopted all 12 months of the Babylonian calendar as their civil calendar. But not all of the 12 months are listed in the Bible. The seven that occur are: Nisan, the first month (Neh 2:1); Sivan, the third month (Est 8:9); Elul, the sixth month (Neh 6:15); Chislev, the ninth month (Zech 7:1); Tebeth, the tenth month (Est 2:16); Shebat, the eleventh month (Zech 1:7); and Adar, the twelfth month (Ezra 6:15). The first month of this calendar also fell during the springtime.
Since Israel was an agricultural society, its calendar worked well for the people and their religious festivals. In the first month (coinciding with our March/April), the fourteenth day was Passover (Ex 12:18); the fifteenth day through the twentyfirst day was Unleavened Bread (Lev 23:6); the sixteenth day was Firstfruits (Lev 23:10-14), dedicating the first-ripe barley sprigs. The second month (April/May) marked the celebration of a later Passover, in case some had missed the first celebration (Num 9:10-11).
On the sixth day of the third month (May/June), the people celebrated Pentecost, which was also called the Feast of Weeks (Lev 23:15-22), in commemoration of the completion of the barley and wheat harvests. In the seventh month (September/ October), the first day was the Feast of Trumpets (Lev 23:23-25; Num 29:1), celebrating the New Year; the tenth day was the Day of Atonement (Lev 16:29-34; 23:26-32); the fifteenth to the twenty-second days were the Feast of Tabernacles or Ingathering (Lev 23:33-43) in commemoration of all the harvests of the year. Thus, the feasts revolved around the harvests.
With regard to the year, the Jewish historian Josephus stated that Israel had two New Years-the commercial New Year, which began in the fall (seventh month), and the religious New Year, which began in the spring (first month). Since the months were based on the lunar system and since each month averaged 29 1/2 days, the year would be 354 days, or 11 days short of the solar year. In just three years the calendar would be off more than a month.
To reconcile the lunar month with the solar year, Babylon had a sophisticated system where seven months would be added to the calendar over a 19-year cycle, resulting in an error of only two hours and four minutes by the end of the cycle.
This is remarkable accuracy for that day. Israel must have adjusted her calendar in a similar fashion by adding a "Second Adar" month whenever necessary.
Between the Testaments - During the period when the Greeks ruled the ancient world, the Seleucid calendar system was most widely used. Two basic systems were used for reckoning time in the Seleucid era-the Macedonian calendar and the Babylonian calendar. It is difficult to be dogmatic as to which system was used, but the Jewish people seem to have used the Macedonian calendar. This means the Seleucid era in Jewish history began on the first day of their seventh month, Tishri, about 15369 AD
In the New Testament - The New Testament contains no references to the Roman or Gentile calendar or to the Jewish calendar, except in speaking of the days of the week. There is also one reference to the "new moon" (Col 2:16). The Sabbath, Saturday, is mentioned about 60 times (for instance, Matt 12:1-12). The New Testament also mentions the "first day," Sunday (Mark 16:2; Luke 24:1; Acts 20:7; 1 Cor 16:2), "the Lord's Day," Sunday (Rev 1:10), and the "Day of Preparation," or "Preparation Day," Friday (Matt 27:62; Mark 15:42; Luke 23:54; John 19:14,31,42). However, these are references to the cultic aspects of the Jewish calendar. Frequent mention is made, especially in the Gospel of John, of the Passover (John 2:13,23; 6:4; 11:55; 12:1; 13:1; 18:39). Other festivals mentioned in the New Testament are Unleavened Bread (Matt 26:17; Mark 14:1,12), Pentecost (Acts 2:1; 20:16; 1 Cor 16:8), Feast of Tabernacles (John 7:2), and the Feast of Dedication (John 10:22).
Although the New Testament makes no references to the Roman or Gentile calendar, it does refer to the reigns of rulers.
The most specific example is Luke 3:1, which speaks of "the fifteenth year of the reign of Tiberius Caesar." This refers to the time of the rulers then in office in Judea and the surrounding territories and to the beginning of the ministry of John the Baptist. This must have been in A.D. 28 AD - 29 AD, assuming that Luke used either the Julian calendar, which began in January, or the regnal calendar, which began in August. The most general references speak not of the year but of the reigns of emperors Caesar Augustus (Luke 2:1) and Claudius Caesar (Acts 11:28), of provincial governors Quirinius (Luke 2:2) and Gallio (Acts 18:12), of King Herod (Matt 2:1; Luke 1:5), and of the ethnarch Aretas (2 Cor 11:32).
One New Testament calendar problem is that the Gospels of Matthew, Mark, and Luke portray Jesus as having celebrated the Passover with His disciples on the eve of His betrayal (Matt 26:19-20; Mark 14:16-17; Luke 22:13-15), whereas the Gospel of John pictures the Jews as not having celebrated the Passover at this time (John 18:28). Many attempts have been made to reconcile this problem.
Possibly, the solution is that the first three gospels reckoned their timetable of the crucifixion events according to the Galilean method (beginning the day at sunrise) which was used by Jesus, the disciples, and the Pharisees. But John may have reckoned according to the Judean method (beginning the day at sunset), a system used by the Sadducees. If this is true, different calendar systems may have been in use at the same time within the nation of Israel.
(from Nelson's Illustrated Bible Dictionary, Copyright (c)1986, Thomas Nelson Publishers)
The Jewish Calendar in Ancient Hebrew History
Bible History Online