The Foundation and Structure of the Hebrew Calendar

The Jewish calendar is based on the lunar cycle. The beginning of the moon’s cycle appears as a thin crescent, which is the signal for the new Jewish month. The moon grows until it is full, marking the middle of the month, and then it begins to wane until it cannot be seen. This entire cycle takes approximately twenty-nine and a half days. It remains invisible for approximately two days, then a thin crescent appears, and the cycle repeats again and again.

Monthly Cycles and Rosh Chodesh

Since a month needs to consist of complete days, a Jewish month is sometimes twenty-nine days long (known as chaser, “missing”), and sometimes thirty (meaning malei, “full”). The first day of the month, as well as the thirtieth day of a malei month, is called Rosh Chodesh, the “Head of the Month,” and has somewhat festive status. Knowing exactly when the month begins has always been important in Jewish practice, because the Torah schedules the Jewish festivals according to the days of the month.

The Solar-Lunar Reconciliation and Leap Years

The Jewish calendar must coordinate with the cycle of the sun and seasons which are determined by the solar orbit. A lunar year, twelve lunar months added together, only adds up to about 354 days, while a solar year, at almost 365.25 days, is around eleven days longer. If no adjustment is made to the calendar, Passover would occur eleven days earlier each year, eventually drifting into winter, then fall, summer, and then spring again.

The solution to reconcile the Jewish Calendar to the solar cycle is to add one thirty-day month about every third year, creating a leap year, referred to in Hebrew as shanah meuberet or "pregnant year." The month is added to the month of Adar, and in these leap years, Adar is referred to as Adar I and Adar II.

Calendar Year Comparisons

Calendar Type	Total Annual Days	Structure
Standard Lunar Year	354 days	Six 29-day and six 30-day months
Solar Year	~365.25 days	Determined by solar orbit
Hebrew Israelite Year	364 days	A true 52 weeks

The Jewish Year and Gregorian Conversion

The Jewish year starts on Rosh Hashanah, "the Head of the Year," the day when Adam and Eve were created. This occurs on the first day of Tishrei in the Jewish Calendar, which coincides with September-October on the Gregorian Calendar. To find the corresponding Jewish year for any year on the Gregorian calendar, add 3760 to the Gregorian number if it is before Rosh Hashanah, and 3761 afterwards.

The Hebrew Israelite Calendar Perspective

On the Hebrew Israelite Calendar, leap weeks seem to fall every sixth even-numbered year of the Gregorian calendar. A leap week is exactly 7 days long and does not count toward the year in which it falls. This means a phantom week will be created after the Spring or Fall equinox of those given years to push the first week of April to the beginning of the Hebrew year, where it belongs. Yah’s year is a true 52 weeks. And 7 days in a week multiplied by 52 equals 364, not 365!

Managing Biblical Calendars Digitally

For those tracking these dates, you can use technical instructions for manually adding calendars. For Google Calendar, open the section labeled "Other calendars," click the plus sign (+), select "Import," and choose the .ics file. For Microsoft Outlook, go to the "File" tab, select "Open & Export," and click "Import/Export" to select the iCalendar file. Apple Calendar users can use the File > Import menu to merge events into their preferred calendar.