The opening Mishna of masechet Megillah teaches that those living in small rural communities could fulfill the obligation to hear the Megillah on the Monday or Thursday prior to Purim. With no one to read the Megillah in their villages, a special Megillah reading was arranged for them when they came to the cities on the market day, i.e., the Monday or Thursday that preceded Purim. Expecting the farmers to come in an additional time was just too onerous.
Yet it is not only for rabbinic holidays that the rabbis were willing to “manipulate” the calendar. Our Sages teach that G-d pointed to the emerging new moon so that Moshe could see the exact point at which the new month should be declared (Menachot 29a). Yet our Sages had little compunction in delaying the declaration of Rosh Chodesh despite the astronomical error involved.
The most famous of these delays is that of lo adu rosh, namely that the first day of Rosh Hashanah can never fall on the first, fourth or sixth day of the week (Sunday, Wednesday or Friday). The Gemara (Rosh Hashanah 20a) presents two possible reasons as to why Rosh Hashanah cannot fall on a Wednesday or Friday. According to Rav Acha bar Chanina, this has little to do with Rosh Hashanah and much to do with Yom Kippur. Having Shabbat and Yom Kippur adjacent to each other would mean that funerals could not be held for two days, leading the body to decompose and violating the respect due the deceased. While rare and generally ill-advised today, funerals may be held on Yom Tov.
On the other hand, Ulla argues that it is due to the “vegetables”. With a two day Yom Tov, vegetables would have to be picked before the first day and thus, would not be fresh on the second. In order to have fresh vegetables, we push off Yom Tov! Such a consideration applies equally to Pesach, and it is likely that the beit din would delay Rosh Chodesh Nissan if need be—the Gemara’s discussion took place when the calendar was not yet fixed, and each month could be dealt with independently.
Combining lo adu rosh with another calendric rule, that of molad zaken—that if the astronomical appearance of the new moon over Jerusalem occurs after 12:00 noon, Rosh Chodesh is delayed until the morrow (or for two days, if the next day would result in Rosh Hashanah being on Sunday, Wednesday or Friday)—close to half the time the first of Tishrei does not occur on its true astronomical date.
These were not the only considerations in deciding whether we should delay the onset of a new month. Our Sages ruled that if witnesses arrived to declare the new moon after the afternoon sacrifice was brought, their testimony would not be accepted and Rosh Hashanah would be delayed to the next day (Rosh Hashanah 30b). As sacrifices were accompanied by the singing of Tehilim by the Levi’im, with a special song for special occasions, if the time for the afternoon sacrifice arrived before witnesses had come, the Levi’im would not know what song to sing. They could not sing the song for Rosh Hashanah, as it would not yet be Rosh Hashanah. But singing the regular weekday song might turn out to be in error if witnesses were to arrive soon thereafter. To avoid the recurrence of this problem, our Sages enacted the law that we should ignore the testimony of the witnesses in such a case and declare Rosh Hashanah to be celebrated on the morrow.
“These are the holidays of the Lord אֲשֶׁר־תִּקְרְא֥וּ אֹתָ֖ם, that you shall proclaim in their time” (Vayikra 23: 44). Our Sages were given the power to set the calendar. And even if they were in error, so be it. “Atem, even if in error; atem, even a willful error; atem, even if you are misled [by false witnesses]” (Rosh Hashanah 25a).
Not only may the month, and hence all our holidays, be declared on the “wrong” day, the Sages had tremendous leeway in deciding which years would have an extra month. Reasons for declaring a leap year included if road or bridge repairs were needed due to the winter rains or if time was needed to fix the ovens that were damaged over the winter, (thereby making it easier to cook for Pesach). The year could also be extended for the benefit of those Diaspora Jews who had begun their travel to Israel for Pesach, but were delayed (Sanhedrin 11a). Our Sages were most concerned not to inconvenience people, and were willing to “manipulate” the calendar to avoid such.
The above makes us wonder what was so wrong with the claim of an “old Boethusian man” that “Moshe Rabbeinu loved the Jewish people, and he knew that Shavuot is only one day. Therefore, he arose and established it after Shabbat, in order that the Jewish people would enjoy themselves for two days” (Menachot 65a). If we can push off Yom tov to have fresh vegetables, surely we can push off Shavuot so we can have two consecutive days to celebrate!
Perhaps we can simply explain that there is a big difference between delaying the new month by one or two days on occasion, or adding a month this year instead of next, as opposed to doing so each and every year. (See our last post for a different approach.)
Rav Yochanan ben Zackai was unimpressed. “It is eleven days’ journey from Horev to Kadesh Barnea by way of Mount Seir (Devarim 1:2). If Moses was a great lover of Israel, why then did he detain them in the wilderness for forty years?”
Eleven days is more than the expected journey from Mount Sinai to the land of Israel. It is also the difference between the standard solar year of 365 days and the lunar one of 354. Perhaps Rav Yochanan ben Zackai was hinting at the fact that calculating the intricacies of the calendar can only take place in the land of Israel, and only by the duly appointed Sanhedrin. Perhaps, in theory, it could have been arranged that Shavuot occur on a Sunday, but that is a decision for the Sanhedrin and not one to be made by those who generally reject rabbinic authority.
Rabbi Yochanan literally saved Judaism during the dark days of destruction of the Temple and subsequent exile. He had witnessed how disunity had almost destroyed the people, and he was determined to ensure that the Jewish people would not splinter into two (or more) factions. There is much about which we can disagree, but we dare not have Jews celebrating holidays on different days.
 The reason Rosh Hashanah cannot fall on a Sunday is to ensure that Hoshana Rabba does not fall on Shabbat. Many Jewish groups rejected the special customs of this day, claiming it was an unwarranted invention of the rabbis. The rabbis feared that if Hoshana Rabba fell on Shabbat, this idea would gain traction.
 As the testimony of witnesses was valid only in the daytime, when the 30th of Elul arrived, one would never know if that day would, in the end, be declared Rosh Chodesh and hence, Rosh Hashanah. Since it very well might, it was to be observed as Rosh Hashanah. If witnesses did arrive, those in close proximity to the Temple (the Sanhedrin was located on the grounds of the Beit HaMikdash) would observe just that one day as Rosh Hashanah; if none arrived, the following day would be Rosh Hashanah. In any case, even after this enactment, where one knew by midday that Rosh Hashanah would not be until tomorrow, “they practiced that day as holy and the next day as holy”. Since few Jews, even in Israel, would know until after Rosh Hashanah which day was actually Rosh Hashanah, the custom developed that Jews in Israel also celebrate two days of Rosh Hashanah.
 This derasha is based on the fact that the Torah writes the word otam, אֹתָ֖ם, without a vav such that it can be read as atem, —that, right or wrong, “you”, i.e., the beit din, declares the beginning of the new month.
 Considering that the latest day to declare a leap year is the 29th day of Adar, a full two weeks before Pesach, it meant that these Jews were dedicating up to six weeks’ time in order to fulfill the mitzvah of aliyah leregel. How could we not delay Pesach for them? Similarly, while Jews in Israel should theoretically begin saying v’ten tal u’matar on Shmini Atzeret, they only begin on the 7th of Cheshvan, so “the last of the travellers can reach Nehar Prat” (Taanit 10a). It is just not nice to pray for rain while those who made the effort to come to Jerusalem are on their way home.
The Melechet Shlomo notes that even after the destruction of the Temple, the custom of waiting until the 7th of Cheshvan was retained. While there was no longer a technical mitzvah of aliyah leregel, nonetheless, many Jews would come to Jerusalem to celebrate the Yamim Tovim. While one might question the historical accuracy of such in the years immediately following the destruction, today we are fortunate that his words ring true.
Nehar Prat was the furthermost border from Jerusalem. Undoubtedly, there were some Jews who travelled from even further, despite the fact that the mitzvah of aliyah leregel did not apply to Jews living outside of Israel (Tosafot Pesachim 3b, s.v. me’aliyah). With rain so crucial and with most people home within 15 days, our Sages felt that they could wait no longer to have people pray for rain, a powerful lesson in the efficiency of prayer.
 Interestingly, had it taken only those 11 days to arrive in Israel, the journey from Sinai to Israel would have taken exactly one year. The people arrived at Sinai on Rosh Chodesh Sivan (see Shemot 19:1), and left “in the second year, in the second month on the twentieth of the month [as] the cloud rose up from over the Tabernacle of the Testimony and the children of Israel travelled on their journeys from the Sinai desert” (Bamidbar 10:10-11). Eleven days brings us to Rosh Chodesh Sivan. This would have paralleled the one year that elapsed from the beginning of the Exodus and the dedication of the Mishkan on the first of Nissan exactly one year later.
 The Rambam (Sefer Hamitzvot #153) goes so far as to claim that if, G-d forbid, there were no Jews living in the land of Israel, the calendar would be invalidated and we would be unable to celebrate the Jewish holidays.
 The way our current calendar is fixed, neither Rosh Hashanah, Yom Kippur, Sukkot nor Shemini Atzeret can fall on a Friday or Sunday. While Pesach can fall on Sunday (but not Friday), it very rarely does. Ironically, it is only the holiday of Shavuot that can semi-regularly (including this past and upcoming year) occur on a Sunday.