The Biblical (Hebrew) calendar was the time keeping tool that Jesus and the early church considered authoritative and used for the observance of God's annual Feast days (Calendar of Christ and Apostles by C. Franklin, revised 2004).
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 season" (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 equivelant to 3 1/3 seconds.
Although the first day of the Biblical (Hebrew) sacred (religious) year is Abib 1, which always falls in the spring, the first day of the civil year is Tishri 1, which occurs in the fall. Tishri 1 is calculated each year based on a set of postponement rules. Once this day is determine then the number of days in each month, as well as the number of months composing the year, can be arrived at following certain rules.
A Biblical year can consist of twelve months (called a common year) or thirteen (an intercalary or leap year). The names of the months, starting in the Spring, are 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).
The Biblical calendar operates based on a repeating 19 year time cycle. Years 3, 6, 8, 11, 14, 17 and 19 of each cycle are considered leap years and have a 13th month, Adar II, added to them. Regular or non-leap years can contain 353, 354 or 355 days (based on how many days are in the months of Heshvan and Kislev). Leap years can be composed of 383, 384 or 385 days.