|Jewish Month||Approximate Secular Date||This Month's Special Dates|
|Menachem Av||July–August||Tisha B'Av|
|Tishrei||September–October||The High Holidays (Rosh Hashanah and Yom Kippur), Sukkot,
Shmini Atzeret, and Simchat Torah
|Tevet||December–January||Conclusion of Chanukah|
"The L-rd spoke to Moses and to Aaron in the land of Egypt, saying, ‘This chodesh shall be to you the head of months.'" (Exodus 12:1–2) From the wording of this verse, "shall be to you," the sages deduced that the responsibility of pinpointing and consecrating the chodesh, the crescent new moon, was entrusted to the leaders of our nation, the Sanhedrin, the rabbinical supreme court of every generation. Originally, there was no fixed calendar. There was no way to determine in advance the exact day of a coming holiday or bar mitzvah, because there was no way to determine in advance when the month would begin. Each month anew, the Sanhedrin would determine whether the month would be 29 or 30 days long—depending on when the following month's new moon was first sighted—and would sanctify the new month.
In the 4th century CE, the sage Hillel II foresaw the disbandment of the Sanhedrin, and understood that we would no longer be able to follow a Sanhedrin-based calendar. So Hillel and his rabbinical court established the perpetual calendar which is followed today.
According to this calendar, every month of the year, except for three, has a set number of days:
Regarding the variable months of Kislev and Cheshvan, there are three options: 1) Both can be 29 days (the year is chaser), 2) both are 30 (the year is malei), or 3) Cheshvan is 29 and Kislev is 30 (the year is k'sidran, meaning these two months follow the alternating pattern of the rest of the months). Hillel also established the rules that are used to determine whether a year is chaser, malei, or k'sidran.
The rules of the perpetual calendar also ensure that the first day of Rosh Hashanah will never take place on Sunday, Wednesday or Friday.6 When Hillel established the perpetual calendar, he sanctified every Rosh Chodesh until Moshiach will come and reestablish the Sanhedrin.