The Hebrew months are named, in order, Abib (Nisan), Iyar, Sivan, Tammuz, Ab, Elul, Tishri, Heshvan, Kislev, Tebeth, Shebat and Adar. The months which always contain thirty days are Abib, Sivan, Ab, Tishri and Shebat. The months of Iyar, Tammuz, Elul, Tebeth and Adar always contain twenty-nine days, with the months of Heshvan and Kislev containing either twenty-nine or thirty days.
Although the first day of the 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 the calendar date on which Feast of Trumpets (Rosh Hashanah) is celebrated. The Hebrew months of Abib (Nisan), Sivan and Tishri contain at least one of God's annual Feast days of worship.
A 'common' Hebrew year is one which contains twelve months. Since the calendar months of Heshvan and Kislev can contain either 29 or 30 days, there exists three different kinds of common years. A 'perfect' common year (where both months have 30 days) contains 355 days. A 'regular' year has 354 days and a 'deficient' one has 353.
A leap (also called intercalary) year contains an additional month. The twelfth month, Adar, is considered Adar I and a thirteenth month, Adar II, is added. The added month is inserted, out of mathematical necessity, to years 3, 6, 8, 11, 14, 17, and 19 of each 19-year lunar time cycle on which the calendar is based. The result of the added month means that a leap year can have either 385, 384 or 383 days.
A Hebrew 'part' is a measurement of time equating to 3 1/3 (3.333 repeating) seconds. This means there are 18 parts to our modern minute (18 x 3.333 seconds = very close to 60 seconds). An hour is divided into 1,080 parts (1,080 x 3.333 = very close to 3,600 seconds).
A Molad is the calculation of the average conjunction of the moon with the earth and the sun. It is based on the mean or average length of the lunar month, which is 29.53059 days. The Molad is not the astronomical conjunction. A Postponement is a one or two-day adjustment to the calculation of the Molad of Tishri. These adjustments enable the process of calculating the Molad to consistently achieve the greatest degree of accuracy in relationship to the cycle of the moon.
Rules for determining God's Feast Days
Postponements are part of the process of calculating and declaring the date of the Feast of Trumpets (Tishri 1, the first day of the Hebrew sacred year), which in turn is used to set the year's dates for observing God's annual Holy Days. After calculating the Molad of Tishri, one of the following four postponement rules are applied to declare Tishri 1.
Rule 1 is invoked when Tishri's Molad or advancement occurs on a Sunday, Wednesday, or Friday. The declaration of Tishri 1 is advanced one day to a Monday, Thursday or Saturday (Sabbath) respectively. This rule prevents the Day of Atonement (Yom Kippur) from falling on a Friday or Sunday. Since Atonement is a complete day of fasting coupled with the command that absolutely NO work is to be done on the day, this rule prevents a fast day followed by a day of feast and rejoicing before God (the Saturday Sabbath) and vice versa.
Rule 2 is invoked when Tishri's Molad happens at or after noon (18 hours 0 parts). The declaration of Tishri 1 is delayed to the following Hebrew day.
Rule 3 is invoked when Tishri's Molad of a common year occurs on a Tuesday (on or after 9 hours and 204 parts). The declaration of Tishri 1 is moved to the next day (Wednesday). Then, because of the above listed Rule One, the day declared as Tishri 1 is moved to Thursday.
Rule 4 is used when Tishri's Molad of a common year comes right after an intercalary year occurs on a Monday, at or after 15 hours and 589 parts. The declaration of Tishri 1 on the Hebrew calendar is advanced to Tuesday. This rule applies roughly once every 186 years.