A lasting festival
ביום השמיני עצרת תהיה לכם
The eighth day will be an atzeres for you
The end of this week’s parsha lists the offerings that are brought on the various festivals throughout the year. The holiday on the eighth day of Sukkos is called Shemini Atzeres, based on the verse describing the holiday as an “atzeres”. What does this term mean? Rashi says that it means to refrain, indicating that we should refrain from creative work on this festival. The verse is teaching a positive commandment. However, we see elsewhere that the word shabbason is used as a positive injunction to refrain from creative activity. If so, why does the Torah change the word it normally uses, and describes this mitzvah as atzeres? Also, if atzeres merely means to refrain from creative activity, this description would also fit the first day of Sukkos, not only the last. It too prohibits these activities. We also see the last day of Pesach is called atzeres, but not the first. Why is this so?
There are those who explain it to mean enclosed and limiting, such that nothing can escape its confines. This is like the verse ועצר את השמים, that “the skies will be blocked off from giving forth rain”. This is fitting for the last day of the festival, which encloses the beginning of its festival. Meaning, with these final days, the festivals come to a close. This explanation is suitable for the final day of Pesach, which is the conclusion of that festival. However, this doesn’t satisfy the festival of Shemini Atzeres. Even though it’s referred to as the eighth day [of Sukkos], it really stands on its own. It is its own independent festival. As such, it can’t be considered the boundary of the festival of Sukkos. These explanations for atzeres still don’t explain why only the final days of the festival have this description.
It would appear that the Torah is trying to teach us something especially practical. That is, people have a special affinity towards acquiring worldly possessions. In order to acquire them, they invest a significant amount of their time working towards the means to acquire these possessions. They work long hours, they go window shopping, they discuss business deals. All of this time could have been spent acquiring heavenly possessions: their eternal reward in the World to Come. Therefore, the Torah’s intent is to free people from this fruitless endeavor and severe time commitment, and exchange it for accepting their Creator’s Will. To accomplish this, the Torah designated specific times of the year for festivals, which have one simple goal: to inspire us to abandon these never-ending quests for more possessions, and to attain only that which is essential. By refraining from creative work, we will be reminded that our goal in this world is to serve our Creator, and that our worldly possessions are here to help us with that goal.
This is why the holidays are called מועדי ה’, the Festivals of Hashem. Meaning, they are designated times, exclusively for spiritual matters. They are the polar opposite of physical endeavor in worldly matters. These special times are to make a great impression on our souls, such that we will despise extras in our physical possessions. This impression will cause us to spend even non-holidays with the proper focus, free from this habituated burden. Our endeavors in a livelihood and in purchasing possessions will be for a spiritual purpose, as opposed to a purely physical one. This is also why the holidays are called מקרא קודש, meaning a day that is designated and prepared for holiness.
Due to this purpose, i.e. to retain even after the festival this lofty perspective which was acquired during the festival, the final day of the festival is referred to as “atzeres“, meaning, a lasting festival. That is to say, a day on which we gather all of our strength to enforce in our souls these lofty lessons, which we were accustomed to all the days of the festival. This is to prevent these lessons from leaving us upon the conclusion of the festival. Instead, they will be retained within us, in order that they be actualized for the rest of the year from the mere potential which they had during the days of the festival.
 Based on HaKesav VeHaKaballah to Numbers 29:35
 Numbers loc. cit.
 ibid 28:16-29:39
 to 29:35, based on Chagigah 18a
 See Mizrachi and Gur Aryeh ad. loc.
 Such as Leviticus 23:39. Targum Onkelos ad. loc. translates it as נייחא. See also Ramban ad. loc. who explains that verse to be teaching a positive commandment
 Deuteronomy 16:8
 HaKesav VeHaKaballah also asks on Rashi’s second explanation (ad. loc., based on Sifrei Bamidbar § 151), that it means refrain from leaving. This refers to staying overnight in Yerushalayim after bringing the Temple offerings. Here too requires explanation why this is only stated with the end of the festival, and not the first day as well. He also asks this on a third explanation Rashi gives in Deuteronomy loc. cit., who explains it to mean a gathering for feasting and drinking
 I couldn’t find who says this
 Deuteronomy 11:17
 He refers to this in German as “בעשלוסס פעסט”, meaning a concluded festival
 Rosh Hashanah 4b
 See Mesillas Yesharim Chapter 1
 Leviticus 23:44
 Exodus 12:16. See the Kesav VeHaKaballah’s commentary to Leviticus 23:3
 In German he refers to this as “אנאלטס-פעסט”
 This approach also explains why Chazal refer to Shavuos as עצרת (for example Rosh Hashanah 4a)