According to the Bible: “In the seventh month, on the first day of the month, shall be a solemn rest unto you, a day of remembrance…” (Lev. 23:24).
The festival was later called Rosh Hashanah, the “New Year for Years” in the Mishnah, and described as a day of judgment.
In the Bible, the Hebrew months are referred to primarily by numbers.
The months received Babylonian names during the exile following the destruction of the First Temple in 586 BCE. These names were retained as a reminder of the redemption, according to the Jewish sage Ramban.
The Babylonian word Tishri apparently derives from the root seru, which means “to begin.” It marks the beginning of the autumn harvest and the commercial year.
“If you check the Mishnah, you will see that the Hebrew calendar has four new years,” noted British computer scientist and calendar expert Edward Reingold.
“They are similar to the school year, the tax year and the fiscal year as we use them today.”
Nissan, the first month of the Hebrew calendar, was known as the New Year of Kings. That’s because the number of years of a king’s reign were counted from that date. In ancient times, legal documents were also dated by the year of the current monarch’s reign.
The New Year of Tithing Animals, putting aside one-10th as a sacrifice, occurred in the month of Elul.
The New Year of the Trees falls on Tu B’Shevat, the 15th of Shevat.
Reingold and Nachum Dershowitz, both professors of computer science at the University of Illinois in Urbana, have spent more than 10 years researching 14 different calendars.
Mathematically intrigued, they developed algorithms to convert the dates of one system to another. Their research is found in their book, “Calendrical Calculations.”
One doesn’t have to be a computer scientist to figure out how the Hebrew calendar works, but it helps. During his recent sabbatical in Israel, Reingold explained some of the intricacies and idiosyncrasies of the Hebrew calendar.
Rosh Hashanah is never on Sunday, Wednesday or Friday. If it falls on these days it is always postponed until the next day.
Excluding Wednesday and Friday prevents Yom Kippur from falling on Friday or Sunday.
There are four reasons why Rosh Hashanah may be postponed, which actually happens 60 percent of the time, making the exception the rule.
The appearance of the new moon (the molad or birth), when the moon is between the earth and the sun, determines the time of the new month.
When the molad occurs at midday or later, the new year is delayed a day. If, for example, this happens on a Tuesday (and because Wednesday is the next day), Rosh Hashanah is delayed two days until a Thursday.
“The astronomical molad varies and therefore we compute an approximate time, an approximate mathematical average,” said Reingold, drawing curves in the air to demonstrate the variation.
The molad that is announced in the synagogue for the blessing on the Shabbat preceding the new month can be in error by as much as a day.
Likewise, the Hebrew calendar isn’t fixed. It can have anywhere from 353 to 385 days, and 12 or 13 months with 29 or 30 days.
The Hebrew day also varies because it is not locked into a rigid hour scheme.
As the length of daylight and nighttime vary with the seasons, the length of daytime and nighttime “hours” also vary with the season. The Hebrew calendar divides the temporal hours into 1080 parts of 3-1/2 seconds each, and each part is divided into 76 moments.
So in London, for example, “the length of such an hour varies from about 39 minutes in December to 83 minutes in June,” said Reingold.
Like the Islamic and Baha’i day, the Hebrew day begins at sunset, whereas a day on the Gregorian, or civil, calendar begins after midnight.
Getting a date right is of major importance in Judaism.
A mistake, 17th century scholar Rabbi Hezekiah de Silo once said, “can cause the holy and awesome fast to be nullified, leaven to be eaten on Passover, and the holidays to be desecrated.”