Throughout rabbinic literature, the Hebrew word אַךְ is understood to be a word of “exclusion” – meaning that whenever it is used, it informs us that though a particular rule broadly applies, it doesn’t apply in all cases.
This is relevant to today’s daf (Pesachim 71a) where we are told with reference to Sukkot that שִׁבְעַת יָמִים תָּחֹג לַה’ אֱלֹהֶיךָ בַּמָּקוֹם אֲשֶׁר יִבְחַר ה’ …וְהָיִיתָ אַךְ שָׂמֵחַ – “For seven days you shall celebrate to God your Lord in the place that God will choose… so that you will only (אַךְ) experience simcha (happiness)” (Devarim 16:15). Yet, if the word אַךְ comes to exclude something relating to the duty of experiencing simcha, what is being excluded?
To answer this question, we must note that the שלמי שמחה was the sacrifice that was offered on each day of the festivals, and that many commentaries explain that the mitzvah of simcha on Yom Tov is associated with the consumption of the שלמי שמחה. But when were the שלמי שמחה offered? They were offered in the daytime, and they were consumed during that day and the following evening.
Given all this, the Gemara teaches us that the word אַךְ comes to inform us that while simcha continues to reverberate on the night following Sukkot (i.e. the night of Shemini Atzeret), it is not truly felt and is “excluded” on the first night of Sukkot. This is because the last night (i.e. Shemini Atzeret) is preceded by simcha, while the first night of Sukkot is not preceded by simcha.
To explain what is really being said here, we must turn to the Shem Mishmuel, written by Rabbi Shmuel Bornsztain (1855-1926) in his insights to Shmini Atzeret (5679) who reminds us that there are two parallel systems of Jewish time.
It is true that the Jewish day begins in the evening as expressed by the order in which the creation is described in the Torah: ‘and it was evening, and it was morning’. However, as the Gemara (Chullin 83a) notes, when it comes to matters of sacrifices, the day begins in the morning and ends in the evening.
What this means is that when it comes to matters that are initiated ‘in heaven’ which then positively impact ‘on earth’ (such as Shabbat), the day begins in the evening, while when it comes to matters that are initiated here ‘on earth’ in order to positively ‘impact’ heaven (such as sacrifices), the day begins in the morning. And since Shemini Atzeret is preceded by simcha (i.e. the offering of the שלמי שמחה), then simcha continues to reverberate on the night of Shemini Atzeret, while the first night of Sukkot, which is not preceded by simcha (i.e. the offering of the שלמי שמחה), does not fully experience the full force of simcha. Interestingly, this insight may explain why many of us enjoy the meals of first night Yom Tov, but don’t fully feel that we are drawing from the simcha energy of Yom Tov until the next day.
Finally, while researching this topic, I came across a fascinating insight offered by Rav Yinun Cohen in his ‘Moadei Kodshecha’ who points out that we do not hold weddings on festivals given the rule of אין מערבין שמחה בשמחה – we do not mix one simcha with another. However, we are also told that weddings can occur on Erev Yom Tov and that the wedding meal can be held on the first night of Yom Tov (see Shulchan Aruch Orach Chaim 546:3). But how could this be so? According to Rav Cohen, the justification for this is the fact that simcha is not fully felt on the first night of Yom Tov (nb. according to the Sha’agat Arieh No. 68, there is only a rabbinic duty of simcha on the first night of Yom Tov), and thus the problem of אין מערבין שמחה בשמחה does not fully apply.