The Jewish year is based on a lunar calendar. The lunar year is not the same length as a solar year so the dates on the Jewish calendar shift in relation to the Gregorian calendar. Months correspond to the 29.5-day lunar cycle and the first day of each month is known as Rosh Chodesh. There are approximately 12.4 lunar months in every solar year which means that lunar years are about 11 days shorter than solar ones which causes the calendar to shift. Many Jewish holidays are directly linked to a particular season or harvest and therefore this drifting needs to be corrected. This is done by the addition of a 13th month, which is known as Adar I. Adar I, if necessary, falls right before the "real" Adar which is then known as Adar II. This calendar adjustment makes the Jewish calendar technically lunisolar. The month names were adopted following the Babylonian exile (586-538 B.C.E) and most names are adaptations from the Babylonian language (Akkadian). They are as follows:
The numbering of years on the Jewish calendar represents the number of years since creation as calculated in the 4th century by computing the literal ages of people in the Torah. This calculation, most likely designed by Hillel II, begins 3760 years before the Gregorian calendar. Dates are written using the Hebrew alphabet. Each letter represents a numerical value and dates are calculated by adding all the letters in a date. Often, dates after 5000 have the 5000 left off of their number (ex: 5775 = 775) therefore, to convert a Hebrew date to a civil one, add 1240. The Jewish year begins on Rosh Hashanah (which literally translates as Head of the Year) and because of this, dates in the months of Tishri, Cheshvan, Kislev, and sometimes Tevet are from the prior civil year. When using the Gregorian calendar, it is Jewish practice to use the nonsectarian B.C.E. (Before the Common Era) rather than B.C. (Before Christ) and C.E. (Common Era) rather than A.D. (Anno Domini).
The Jewish day begins at nightfall and continues until nightfall Because of this, Jewish holidays begin the night before the date on the civil calendar. Except for Shabbat, the days of the week on the Jewish calendar do not have names but rather designations of their place in the week. The week begins with Sunday which is known as Yom Rishon (first day) and finishes with Yom Shabbat (Sabbath day). The length of some major festivals mentioned in the Torah varies depending on where the observance is taking place with only one day being observed in the state of Israel and two days in the Diaspora.
The Jewish year is punctuated by a series of holidays, some part of the Jewish tradition for over a millennium and others only recently integrated into Judaism. In addition to annual festivals, the Sabbath is welcomed weekly and the new moon is observed each month. Celebrations and practice have evolved throughout the ages and can vary depending on the geographic origin or specific tradition of the celebrants. The Jewish Heritage Collection has a large collection of texts that provide additional information about these holidays and their associated objects. Many objects associated with these celebrations have acquired an artistic character, becoming highly ornamented and decorated and the JHC has a vast array of these objects which highlights the diversity and artistic variety of Jewish ceremonial and ritual objects.