The following is a public service announcement from the Freehof Blog.

If you’re thinking of having one last, lovely dinner in the sukkah tonight, October 4, 2015, which marks the onset of Sh’mini Atzeret, you should think  twice. In fact, in this case it might help to think more than twice. (And no, we’re not talking about all the rain currently pounding the east coast of the United States.)

If you live in Israel, or if you are a liberal Jew who does not observe the second festival day that is customary in the Diaspora (yom tov sheni shel galuyot), then according to halakhah you should not eat in the sukkah tonight or tomorrow. The reason, of course, is that the mitzvah is to dwell in the sukkah for seven days (Lev. 23:42), and to add to that period by sitting in the sukkah on Sh’mini Atzeret, which is not part of that seven-day period, would be to violate the principle of bal tosif (בל תוסיף), “do not add to the instruction that I give you” (Deut. 4:1 and elsewhere).[1] So the best thing to do is to eat inside the house.

BUT… if you live in the Diaspora and hold to the traditional observance of a second day of Yom Tov, you do eat in the sukkah tonight and tomorrow. This is due to the principle of s’feika d’yoma (ספיקא דיומא), the “doubt” or uncertainty over the precise calendar date that obtains, according to traditional thinking, everywhere outside the land of Israel. If you follow this principle, then you are officially in doubt as to whether tonight is the eighth day of the festival season – i.e., Sh’mini Atzeret – or the seventh day, i.e., Hoshanah Rabah. If tonight is in fact Hoshanah Rabah, then it’s still Sukkot and we’re still obligated to dwell in the sukkah. The remedy for this doubt follows the classic pattern of adopting both opinions, just to be safe: we eat in the sukkah, just in case tonight is really Hoshanah Rabah, but we don’t say the benediction אשר קדשנו במצוותיו וציונו לישב בסוכה (…Who… has commanded us to dwell in the sukkah), just in case it’s really Sh’mini Atzeret.[2]

You might well ask: if we are in doubt as to the last day of Sukkot, then we should be in the same doubt as to the first day;[3] why then do we recite the blessing leishev basukkah when we sit in the sukkah on the first festival day? For that matter, if we are in doubt as to the correct date for yom tov, so that we have to add a second day just to make sure we’re observing the festival on the proper day, it would follow that we should observe a second day of Yom Kippur, yet – obviously – we don’t.[4] And if the folks in Eretz Yisrael, with whom the rest of us are in constant communication, are not in doubt as to the correct date, why is s’feika d’yoma still a thing? Why do we continue to act as though we are in doubt when it’s clear to everybody that we do know what day it is?

In our next post, we’ll survey the history of these questions and of the contribution of progressive halakhah on the subject of yom tov sheni. In the meantime, if you were thinking of having that last festival meal under the skhakh tonight, our advice to you is to adopt the progressive halakhic conclusion… and that you just back away from the sukkah!

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[1] B. Rosh Hashanah 28a: תניא, מניין לכהן העולה לדוכן שלא יאמר הואיל ונתנה לי תורה רשות לברך אתישראל אוסיף ברכה משלי, כגון ה’ אלהי אבותיכם יוסף עליכם ככם אלף פעמים (פ’ דברים) ת”ל לא תוסיפו על הדבר

[2] If so, and there is no mitzvah tonight to eat in the sukkah, then the b’rakha is an unnecessary one (b’rakha l’vata;a)

[3] I.e., is it 15 Tishri, the first day of Sukkot, or 14 Tishri, erev Sukkot?

[4] The authorities are aware of this glitch in the logic of the observance. Isserles (Oraḥ Ḥayyim 624:5) says that we ought not to observe a second day of Yom Kippur because it is dangerous to fast for 48 hours. But Magen Avraham (note 7, end, and after him Mishnah B’rurah note 17) address the major point: since in fact we know how to determine the correct dates for all the festivals, then we are not truly in doubt as to the date of Yom Kippur and have no real need to fast a second day “just in case.”