The Biblical (Hebrew) calendar is the inspired method for determining when God's seven annual Feast days should be kept. It was the calendar that Jesus, the apostles, and the early church considered authoritative concerning when to keep such celebrations as Passover, Pentecost, and the Feast of Trumpets (Calendar of Christ and Apostles by C. Franklin, revised 2004).
Type of Calendar
The Biblical Calendar is considered one that is lunisolar, meaning that its rules attempt to harmonize the movements of both the sun (for seasons) and moon (for months). This must be achieved since God's annual Feast days must be observed in their proper seasons (Exodus 13:10, Numbers 9:2 - 3, etc.).
Each day of the calendar begins at sunset. This means that the weekly seventh day Sabbath starts each Friday at sunset and ends with sunset Saturday. A Biblical day has 24 hours in it like our modern method of timekeeping but each hour is divided into 1,080 parts unlike our system of 3,600 seconds (60 minutes times 60 seconds). This means that a Hebrew "part" is roughly equivalent to 3 1/3 seconds.
First Sacred and Civil Months
Although the first day of the Biblical sacred year is Abib (Nisan) 1, which always falls in the spring, the first day of the civil year is Tishri 1, which occurs in the fall. Tishri 1 determines when a Biblical year has ended and a new one has begun. This day can be delayed up to two days each year due to the calendar's postponement rules.
Year Length
An individual "normal" Biblical year can contain 353, 354 or 355 days while leap years can possess 383, 384 or 385 days.
The length of a given year can first be affected depending on whether the months of Heshvan and Kislev have either 29 or 30 days. The second factor lengthening the year is whether a leap month is added.
The Biblical calendar relies on a repeating time cycle of 19 years to determine which years are leap years. In years 3, 6, 8, 11, 14, 17 and 19 of the 19-year time cycle a leap month of 30 days is added.
From 3761 B.C. to 2024 A.D., a Biblical year has been comprised of 353 days 581 times. It has contained 354 days 1,406 times and 355 days 1,667 times. In regard to leap years, they have been composed of 383 days 895 times, 384 days 303 times and 385 days 933 times.
Name and Number of Months
A "normal" or common Biblical year has twelve months. These months, starting with the first sacred month in the spring, are called Abib (Nisan) with 30 days, Iyar (29 days), Sivan (30 days), Tammuz (29 days), Ab (30 days), Elul (29 days), Tishri (30 days), Heshvan (29 or 30 days), Kislev (29 or 30 days), Tebeth (29 days), Shebat (30 days) and Adar (29 days).
In a leap year, a 30 day month after Shebat is inserted called Adar I. When this happens the month of Adar, which always has 29 days, is renamed Adar II.