What are the connotations of "Ki Yevarech'cha Hashem Elokecha"?
Seforno: It means thaat Hashem will bless you with a bountiful harvest, and the poor will also benefit from the additional Matanos.
Hadar Zekenim: It means that Hashem will bless you earlier in the summer in your toil, so that you will be able to rejoice on Sukos.
Having written "Vesamachta be'Chagecha" in the previous Pasuk, why does the Torah add here "ve'Hayisa Ach Same'ach"?
Rashi #1 and Targum Yonasan: It is not a command, but a promise - If you will rejoice on Yom-Tov, you will be happy throughout the year (with your successes - Targum Yonasan). 1
Rashi #2: To include the night of Shemini Atzeres in the Din of Simchah. 2
Seforno: You will know only happiness, a happiness that is totally devoid of sadness.
Ba'al ha'Turim: Refer to 16:11:1:1.
Sukah, 45a: To preclude the first night of Yom-Tov from the Din of Simchah. 3
Hadar Zekenim #1: To warn against haughtines 4 - turning one's hearts to idleness.
Hadar Zekenim #2: You have nothing to engage in other than Simchah, because everything is gathered in your hand (houses).
See Na'ar Yonasan.
Refer to 16:15:151:1
Since the word "Ach" always comes to exclude. See Torah Temimah, citingSukah, ibid. and note 68. Refer also to 16:14:1:5.
Hadar Zekenim (citing Kidushin 81a): The worst time of the year is Yom-Tov, since the heart becomes haughty and it is easy to think [about sin] - which is why the Chachamim instigated the Minhag to fast on BeHaB (on the Monday, Thursday and Monday following Pesach and Sukos.
QUESTIONS ON RASHI
Rashi writes that "Vehayisa Ach Same'ach" includes the night of Shemini Atzeres for Simchah. But "Ach" always comes to exclude?
Sukah 48a: "Ach" comes to draw a distinction - to include the night of Shemini Atzeres in the Din of Simchah - since there is Simchah before it, but to exclude the first night of Yom Tov, since there is no Simchah before it. 1
Hadar Zekenim: "Vehayisa" and "Ach" are both exclusions. And an exclusion followed by an exclusion comes to include.
Moshav Zekenim: "Ach" excludes the first night of Yom Tov - and we include the night of Shemini Atzeres, since "Vehayisa Ach Same'ach" is superfluous - seeing as the Torah already wrote in Pasuk 14 "Vesamachta be'Chagecha". We therefore include the latter - since there is aSimchah before it - , and exclude the former, where there is not. 2
Divrei Eliyahu: During the seven days of Sukos, there are other Mitzvos to rejoice in - Sukah, Lulav, Esrog, Hadas, Aravah, and Simchas Beis ha'Sho'eivah. On Shemini Atzeres, the only Mitzvah is Simchah on the Chag itself. Therefore it refers to the night of Shemini Atzeres, from which all the other Mitzvos is precluded.
Refer to note on answer #3.