דבר אל-בני ישראל לאמר בחדש השביעי באחד לחדש יהיה לכם שבתון זכרון תרועה מקרא-קדש: אך בעשור לחדש השביעי הזה יום הכפרים הוא מקרא-קדש יהיה לכם וגו’ דבר אל-בני ישראל לאמר בחמשה עשר יום לחדש השביעי הזה חג הסכות שבעת ימים ליקוק: אך בחמשה עשר יום לחדש השביעי וגו’ ולקחתם לכם ביום הראשון פרי עץ הדר כפת תמרים וענף עץ-עבת וערבי נחל וגו’
Tell the Children of Israel, saying: “In the seventh month, on the first of the month, it shall be for you a day of rest. A remembrance of shofar blasts, a holy convocation. However, on the tenth of this seventh month, it is Yom Kippur. It shall be for you a holy convocation…” Tell the Children of Israel, saying: “On the fifteenth of this seventh month, [it is] the festival of Sukkos, seven days for Hashem. However, on the fifteenth of the seventh month…you shall take on the first day a beautiful fruit, palm fronds, braided branches, and willows…”
If we examine the description of the holidays, we’ll notice a strange inconsistency. The month of Tishrei contains many festivals. First there’s Rosh Hashanah, then Yom Kippur, and then Sukkos. The Torah specifies that these holidays occur in the seventh month of the Jewish calendar. Rosh Hashanah is on the first, Yom Kippur is on the tenth, and Sukkos starts on the fifteenth. When the Torah refers to Yom Kippur, it specifies that it is in this seventh month. For Sukkos, it first says the same as Yom Kippur, this seventh month. The second time it refers to Sukkos, in the context of the mitzvah of the four species, it just says the seventh month. Why is there this inconsistency?
Our Sages ask an interesting question. The Hebrew term for a widow is an almanah. They ask why is she called an almanah? They answer that the Sages enacted that if a widow were to remarry and either be divorced or re-widowed, her kesubah would entitle her to a manah from her husband’s estate. This amount of money is a set amount, entitled to all widows. Even though the Sages enacted this after the Torah was written, Hashem knows the future. He alluded to this enactment in the Torah by referring to a widow as an almanah. Does this phenomenon happen anywhere else in the Torah?
Approximately two millennia ago, the Sages decreed that we are to refrain from blowing the Shofar if Rosh Hashanah falls on Shabbos. There was a concern that if people would be allowed to blow as normal, they would take their Shofar to an expert for advice on how to blow it. If the person would carry the Shofar from one house to another, they could end up transgressing Shabbos. Our Sages saw this prohibition alluded to in the Torah itself. In once place the Torah refers to Rosh Hashanah as a day of Shofar blasts. In another place it says that Rosh Hashanah is a remembrance of Shofar blasts. One is referring to a year when we blow the Shofar on Rosh Hashanah, and once is when we don’t, since Rosh Hashanah is on Shabbos.
Now, the verses we started with refer to Rosh Hashanah as a remembrance of Shofar blasts. That means it’s referring to a year where Rosh Hashanah is on Shabbos. In such a year, Yom Kippur will fall out on a Monday. Its observance is no different than any other year, so the Torah says this seventh month will have a normal Yom Kippur. When Rosh Hashanah is on Shabbos, that means the fifteenth, the first day of Sukkos, will also be on Shabbos. When it comes to the mitzvah of sitting in the sukkah, nothing changes. The Torah therefore says the fifteenth of this seventh month will be the festival of Sukkos.
However, the mitzvah of the four species on Sukkos is different. Just like blowing the Shofar, the Sages decreed not to take the four species on Shabbos, in case the person carry them to an expert to learn how to do the mitzvah properly. As such, in a year where Rosh Hashanah falls on Shabbos, we don’t take the four species on the first day of Sukkos, as it also falls on Shabbos. The Torah therefore can’t say on the fifteenth of this seventh month, take the four species, as that wouldn’t be true. This seventh month is referring to when Rosh Hashanah falls on Shabbos. In such a year, the four species will not be taken. That’s why it simply says on the fifteenth of the seventh month. Meaning, the mitzvah occurs on other years, but not on a year that the Torah is describing. Even though this decree didn’t yet exist when the Torah was written, the Torah has no problems alluding to the future. Indeed, everything is alluded to in the Torah.
 Based on Oneg Yom Tov al Drush Likkutim MeChiddushei Torah s.v. בפ’ אמור דבר אל ב”י
 Leviticus 23:24,27,34,39,40
 The Oneg Yom Tov doesn’t seem bothered why Rosh Hashanah doesn’t say this seventh month. Perhaps since it’s the first time the seventh month is mentioned in the verses. He would expect all the instances that follow to say “this”
 Kesubos 10b
 Rosh Hashanah 29b
 Numbers 29:1
 Leviticus 23:24
 The gemarra ad. loc. initially suggested this was a biblical prohibition, as it seems evident from the verses. It’s hard to understand why it would be biblically prohibited to blow the Shofar on Shabbos, as the gemarra itself points out there’s no melacha involved. What’s worse is the Yerushalmi Rosh Hoshanah 4:1 seems to conclude it’s biblically prohibited. The Shulchan Aruch Orach Chaim 582:6 brings the custom to mention זכרון תרועה when Rosh Hashanah is on Shabbos, as we don’t blow the Shofar. The Biur HaGra ad. loc. writes רא”ש סוף ר”ה וגמרא שם כט ב’ ועי’ ירושלמי שם. I am at a loss to understand why the Biur HaGra cites the Yerushalmi. See Damesek Eliezer ad. loc. See also Mirkeves HaMishnah Hilchos Shofar 2:8 who tries to read the Bavli’s understanding into the Yerushalmi
 Rosh Hashanah loc. cit.; Sukkah 43b
 ליכא מידי דלא רמיזא באורייתא. This aphorism is quoted by many authorities, but doesn’t seem to appear explicitly in Chazal. It seems to be an extrapolation from Ta’anis 9a. See http://forum.otzar.org/viewtopic.php?t=16258