In order to better understand how the Jewish feasts are celebrated it is beneficial to understand how time is measured from a Jewish religious viewpoint.
The Jewish Day
In Genesis 1 God refers to the day as consisting of “the evening and the morning”. The Jewish day is consistent with this, first evening or night hours and then morning or daylight hours. Therefore, the Jewish day begins at sundown and continues to the next sundown. The Jewish holidays also begin at sundown.
For example, this year Passover begins at sundown April 12th (yesterday) and ends at sundown, April 13th (today).
The Jewish Week
The Hebrew week consists of seven days similar to our traditional calendar, however they do no have specific names for the days of the week. They are known as “the first day”, “the second day” and so forth. The seventh day, however is known as the Sabbath or a day of rest. This originates in Genesis when the Lord created the world in six days and rested on the seventh day, not because he was tired (God never grows tired) but because he was satisfied with His work. The seventh day or Sabbath falls on our calendar as sundown on Friday to sundown on Saturday.
The Jewish Month
The Hebrew calendar is based on a lunar month. The month begins with each new moon and therefore usually alternates between 29 and 30 days. The first day of every month is known as Rosh Hodesh (the Head of the Month) and was considered a day of rest. It is considered to be a minor holiday.
The Jewish Year
The Hebrew calendar is based on a compromise between a lunar and solar reckoning (our current calendar is based largely on a solar reckoning). The average length of a month is 29 ½ days. The normal Jewish year consists of 12 months or approximately 354 days. Since the holidays were to be kept within the appropriate seasons and this lunar reckoning is 11 days less than our solar reckoning (which is the basis of the seasons) a compromise was needed to avoid the holidays being celebrated in the wrong seasons after the passing of the years.
Therefore, the Hebrew calendar is based on a 19 year cycle, in which there are leap years the third, sixth, eighth, eleventh, fourteenth, seventeenth and nineteenth years. During the leap years an extra day is added to the month of Adar and a thirteenth month (29 days known as Adar Sheni) is added to the calendar. This is why the Jewish holidays appear to jump around on our traditional calendar. Passover tends to fall somewhere in March/April and the fall feasts occur in September/October.
Since the Hebrew’s exodus from Egypt the new year has begun with Passover (March/April on our calendar). The Hebrews celebrated two New Years, the religious new year began with Passover and the agricultural new year began after the harvest.
The following are the month of the Jewish calendar (based on the religious New Year)
1. Nisan: 30 days
2. Iyar: 29 days
3. Sivan: 30 days
4. Tammuz: 29 days
5. Av: 30 days
6. Elul: 29 days
7. Tishri: 30 days
8. Heshvan: 29 or 30 days
9. Kislev: 29 or 30 days
10. Tevet: 29 days
11. Shevat: 30 days
12. Adar: 29 days (30 days in leap years)