Why are holidays celebrated an extra day in the Diaspora?

The Hebrew calendar…months of the year and days of the week…

The answer has its roots in the history of the Jewish calendar. Originally, there was no fixed calendar. Each month anew, the Sanhedrin (Rabbinic Supreme Court) in Jerusalem would determine whether the previous month had been 29 or 30 days long -- depending on when the new moon of the following month was first sighted -- and would accordingly sanctify the new month.  There was no way to determine in advance the exact day of a coming festival, because every festival falls on a particular day in a month, and there was no way to determine in advance when the month would begin.  The way that the Sanhedrin determined the new month is fascinating in itself: The Lunar Cycle

The Jewish calendar is based on lunar cycles.1 Towards the beginning of the moon’s cycle, it appears as a thin crescent. That is the signal for a new Jewish month. The moon grows until it is full, the middle of the month, and then it begins to wane until it cannot be seen. It remains invisible for approximately two days2—and then the thin crescent reappears, and the cycle begins again.

The entire cycle takes approximately 29½ days.3 Since a month needs to consist of complete days, a month is sometimes twenty-nine days long (such a month is known as chaser, “missing”), and sometimes thirty (malei, “full”).

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 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 semi-festive status. See Why is Rosh Chodesh sometimes one day and sometimes two?

The Jewish Months

Nissan is the first month on the Jewish calendar. Before the Jews left Egypt, on the first day of the month of Nissan, G‑d told Moses and Aaron: “This chodesh (new moon, or month) shall be to you the head of months.”4 Thus the peculiarity of the Jewish calendar: the year begins on Rosh Hashanah, the first day of the month of Tishrei (the anniversary of the creation of Adam and Eve), but Tishrei is not the first month. Rosh Hashanah is actually referred to in the Torah as “the first day of the seventh month.”

The Lunar Cycle

The Jewish calendar is based on lunar cycles. Towards the beginning of the moon’s cycle, it appears as a thin crescent. That is the signal for a new Jewish month. The moon grows until it is full, the middle of the month, and then it begins to wane until it cannot be seen. It remains invisible for approximately two days²—and then the thin crescent reappears, and the cycle begins again.

The entire cycle takes approximately 29½ days.³ Since a month needs to consist of complete days, a month is sometimes twenty-nine days long (such a month is known as chaser, “missing”), and sometimes thirty (malei, “full”).

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 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 semi-festive status. See Why is Rosh Chodesh sometimes one day and sometimes two?

The Jewish Months

Nissan is the first month on the Jewish calendar. Before the Jews left Egypt, on the first day of the month of Nissan, G‑d told Moses and Aaron: “This chodesh(new moon, or month) shall be to you the head of months.”⁴ Thus the peculiarity of the Jewish calendar: the year begins on Rosh Hashanah, the first day of the month of Tishrei (the anniversary of the creation of Adam and Eve), but Tishrei is not the first month. Rosh Hashanah is actually referred to in the Torah as “the first day of the seventh month. 

The Jewish Months and their Special Dates

Jewish Month	Approximate Secular Date	This Month’s Special Dates
Nissan	March–April	Passover
Iyar	April–May	Lag B’Omer
Sivan	May–June	Shavuot
Tammuz	June–July
Menachem Av	July–August	Tisha B’Av
Elul	August–September
Tishrei	September–October	The High Holidays (Rosh Hashanah andYom Kippur), Sukkot, Shmini Atzeret, andSimchat Torah
Marcheshvan	October–November
Kislev	November–December	Chanukah
Tevet	December–January	Conclusion of Chanukah
Shevat	January–February	Tu B’Shvat
Adar	February–March	Purim

Mo. # Babylonian Calendar Meaning of Babylonian Months Theme of Month (Babylonian & Biblical) Biblical Calendar Meaning of Biblical Months (Harvesting Schedule) Biblical Feasts 1 Nisan Their flight Redemption, Miracles Abib Ripening of grain[Barley, Wheat] Passover, Unleavened Bread, Firstfruits 2 Iyyar (natural) healing Introspection, Self improvement Ziv Splendor orRadiance[Flowers] (Barley harvest)    3 Sivan Bright - their covering Giving of Torah 3rd (Wheat harvest) Feast of Weeks (Pentecost) 4 Tammuz Hidden - giver of the vine (A Phoenician deity) Sin of the Golden Calf, guarding of the eyes 4th       5 Av Father Av the Comforter 5th       6 Elul A vain thing - nothingness Repentance 6th (Fruit harvest)    7 Tishri Beginning (from reishit) Month of the Strong or Month of the Ancients Ethanim Ever-flowing streams Rosh HaShannah, Yom Kippur, Succot 8 Kheshvan Eighth The Flood (of Noach) Bul Produce (in the sense of rain)    9 Kislev Security, trust (Restful) Sleep 9th       10 Tevet Good (from "Tov") Divine Grace 10th       11 Shevat meaning unknown Tree of Life 11th       12 Adar Strength Good Fortune 12th       13 * Adar II

The days of the week

Hebrew Weekdays

Except for the Sabbath day, the individual days of the week have no names, just numbers.  The middle column in the table below gives the actual transliterated pronunciation of the names. Yom is pronounced "Yome".  The "Kh" is a guttural sound often spelled as "Ch".  There is no "Ch" sound in Hebrew as there is in English.  Modern Hebrew, however, can create a "Ch" sound by putting an accent mark ( ´ ) in front of the Hebrew letter "Khet" (or "Chet").  This is used only in rare cases in order to properly pronounce such words or names as Church or Churchill (referring to the person Winston Churchill.)

Secular Weekday Name Hebrew "Name" Hebrew Meaning Sunday Yom Reeshone First day Monday Yom Shaynee Second day Tuesday Yom Shlee´shee Third day Wednesday Yom Revee´ee Fourth day Thursday Yom Khah´mee´shee Fifth day Friday Yom Ha´shee´shee Sixth day Saturday Shabbat Rest

Hebrew	Transliteration	English
	Yom Rishon	First Day (Sunday)
	Yom Sheini	Second Day (Monday)
	Yom Shlishi	Third Day (Tuesday)
	Yom R'vi'i	Fourth Day (Wednesday)
	Yom Chamishi	Fifth Day (Thursday)
	Yom Shishi	Sixth Day (Friday)
	Yom Shabbat	Sabbath Day (Saturday)