The Jewish, Julian, and Gregorian Calendars Demystified
The Jewish Calendar
Various calendars have been developed throughout history to mark the passage of time and are commonly based on the solar calendar (the secular calendar), the lunar calendar (the Muslim calendar), or a combination of the two (the Jewish calendar).
Molad, the new moon, defines the start of the new month. A lunar month lasts 29 days, 12 hours, 44 minutes, and 3 1/3 seconds, or 29 days, 12 hours, 793 halaqim. There are between 12 and 13 lunar months in a solar year.
Because 19 years encompass almost exactly 235 months, the Jewish calendar employs a cycle of 19 years, as do the Chinese calendar and the Ancient Greek calendar. In a cycle of 19 years, the Jewish calendar includes 12 common years of 12 months, and 7 leap years of 13 months. Leap years occur in years 3, 6, 8, 11, 14, 17, and 19 of each 19-year cycle.
The months in the Jewish calendar are:
Tishri -30 days
Heshvan – 29 days
Kislev – 30 days
Tevret – 29 days
Shevat – 30 days
Adar 1 – 30 days (Adar 1 occurs only in leap years)
Adar 2 – 29 days
Nisan – 30 days
Iyyar – 29 days
Sivan – 30 days
Tamuz – 29 days
Ab – 30 days
Elul – 29 days
However, a common year is 8 hours, 876 halaqim too short and a leap year is 2 hours, 491 halaqim too long.
Thus, the first day of Tishri will drift from the molad of Tishri (the new moon that marks the beginning of the month of Tishri). If the molad falls on the first day of Tishri, everything is fine. If the first of Tishri arrives too soon, the Jewish calendar adds one day to the preceding month of Heshvan (this is called a complete year). If the first day of Tishri arrives too late, the Jewish calendar subtracts a day from the preceding month of Kislev (this is called a defective year).
In a defective year, Heshvan is 29 days, Kislev is 29 days, a common year is 353 days, and a leap year is 383 days.
In a normal year, Heshvan is 29 days, Kislev is 30 days, a common year is 354 days, and a leap year is 384 days.
In a complete year, Heshvan is 30 days, Kislev is 30 days, a common year is 355 days, and a leap year is 385 days.
But here is where things get more complicated. There are four rules that further govern the Jewish calendar.
If molad Tishri falls on a Sunday, Wednesday, or Friday, the first day of Tishri must be delayed by one day. If Tishri begins on a Wednesday or Friday, Yom Kippur would fall on a Friday or a Sunday, making it impossible to prepare food for the day after the Yom Kippur fast, due to Sabbath restrictions. if Tishri begins on a Sunday, the seventh day of Succoth would fall on the Sabbath.
If molad Tishri occurs at noon or later, the first day of Tishri must be delayed by one day in order to ensure that the new crescent moon is visible during the first day of the year.
If molad Tishri falls on a Tuesday at 3:11:20 AM or later, then the first of Tishri is delayed by one day, otherwise both rules 1 and 2 will be invoked, making the previous year too long (356 days).
If molad Tishri after a leap year falls on Monday at 9:32:43 1/3 AM or after, then the first of Tishri is delayed by one day, otherwise the previous leap year would have been delayed by both rules 1 and 2, making the previous leap year too short (382 days).
Biblical creation began on the 25th of Elul in year 1 and ended on the 1st of Tishri in year 2. Today, May 11, 2008 is the 6th of Iyyar in the year 5768. The conversion from the Gregorian calendar, which we use today to the Jewish calendar, can be computed for Jewish Calendar Conversions.
The Jewish calendar slowly creeps forward compared to the astronomical year since a year lasts 365.2422 days, but the Jewish calendar, on average, lasts 365.2468 days. Thus, the Jewish calendar will creep one day forward every 217 years. The Gregorian calendar which is in common use today is also subject to calendar creep, but the Gregorian calendar creeps forward only 1 day every 3333 years. By comparison, the Julian calendar, which was replaced by the Gregorian calendar, crept forward 1 day every 128 years.
The Julian Calendar
In 45 B.C Julius Caesar ordered a calendar consisting of twelve months based on a solar year. This calendar employed a cycle of three years of 365 days, followed by a year of 366 days (leap year). When first implemented, the "Julian Calendar" also moved the beginning of the year from March 1 to January 1. However, following the fall of the Roman Empire in the fifth century, the new year was gradually realigned to coincide with Christian festivals until by the seventh century, Christmas Day marked the beginning of the new year in many countries.
By the ninth century, parts of southern Europe began observing first day of the New Year on March 25 to coincide with Annunciation Day (the church holiday nine months prior to Christmas celebrating the Angel Gabriel's revelation to the Virgin Mary that she was to be the mother of the Messiah). The last day of the year was March 24. However, England did not adopt this change in the beginning of the New Year until late in the twelfth century.
Because the year began in March, records referring to the "first month" pertain to March; to the second month pertain to April, etc., so that "the 19th of the 12th month" would be February 19. In fact, in Latin, September means seventh month, October means eighth month, November means ninth month, and December means tenth month. Use of numbers, rather than names, of months was especially prevalent in Quaker records.