Chanukah Eighth Night (Zos Chanukah)
The Sfas Emes explains that Zos Chanukah indicates something very fundamental about Chanukah. The Chanukah story begins with the persecution of the Jews by the Assyrian Greeks. The situation was bleak indeed. The gentile rulers were powerful. How could we overcome them? From the depths of this darkness came the salvation.
The word zos/this, is laden with symbolism. In the Zohar we find the word zos alluding to Jerusalem and the kingdom of Heaven. The Midrash says that the word zos/this in the pasuk, “... בְּזֹאת אֲנִי בוֹטֵחַ/… in this I trust,” (Tehillim 27:1) is an allusion to God – in God I trust.
The early kabbalists teach that zos alludes specifically to that point of spirituality through which God gives existence to the physical. David HaMelech as well, asked God to preserve this recognition of His presence within us, “... שָׁמְרָה־זֹּאת לְעוֹלָם לְיֵצֶר מַחְשְׁבוֹת לְבַב עַמֶּךָ .../… Preserve this forever – the product of the thoughts of Your people’s hearts …” (Divrei HaYamim 1 29:18)
Zos, then, is a reference to the Godliness hidden within us and all of Creation. Knowing that everything, including God’s obscurity is powered by this point of God given spirituality, essentially, knowing that God is “in” everything and that everything is therefore “good” is a tremendous tool for strengthening one’s faith particularly in times of exile. This, in fact, is the fundamental meaning of the pasuk in Eicha (3:21), “זֹאת אָשִׁיב אֶל־לִבִּי עַל־כֵּן אוֹחִיל/This I will bear in mind; therefore I have hope.” The prophet is teaching us that when we bear in mind zos – that the exile as well is from God and that He is present even in the darkness of it – we have good reason for hope.
This concept is the lesson of Chanukah. The salvation came when the nation realized that God was with them in the darkness as well. We find this idea as well in the following pesukim from Tehillim (112:7-8), “מִשְּׁמוּעָה רָעָה לֹא יִירָא נָכוֹן לִבּוֹ בָּטֻחַ בַּה': סָמוּךְ לִבּוֹ לֹא יִירָא עַד אֲשֶׁר־יִרְאֶה בְצָרָיו/He will have no fear of evil tidings; his heart is firm, confident in God. His heart is steadfast, he shall not fear, he will even [expect to] see [vengeance upon] his tormentors.” When a person trusts in God, he knows that salvation is at hand. The Chiddushei HaRim points out that, significantly, the last letters of the words, “נָכוֹן לִבּוֹ בָּטֻחַ בַּה׳ סָמוּךְ/his heart is firm, confident in God, steadfast” spells out חֲנוּכָּה/Chanukah.
The chapter in Tehillim that we say on Chanukah bears out this idea. “הָפַכְתָּ מִסְפְּדִי לְמָחוֹל לִי פִּתַּחְתָּ שַׂקִּי וַתְּאַזְּרֵנִי שִׂמְחָה/You have transformed my lament into dancing for me; You undid my sackcloth and girded me with happiness.” (Tehillim 30:12) The word for transformed – הָפַכְתָּ – also means to overturn or to turn inside out. The difference between lament and dancing is whether God is hidden or revealed. Dancing is lament turned inside out, as it were. The important point is that God is present in both. The Midrash on the curses in parshas BeChukosai makes this point when it says that the difference between the blessings and the curses is that the blessings are in the order of the Hebrew alphabet whereas the curses are backwards.
The second half of the pasuk continues this idea. It is important to understand that the sackcloth, a clear reference to exile and God’s concealment is only a cover. When the sackcloth is undone, when the concealment is removed, God is revealed in the form of salvation and closeness to Him. Then we are girded with happiness. This is the meaning of Chanukah, the days of miracles, when the nation of Israel was at a very low point and God helped us. It is encouraging to know that according to the extent of concealment so is the extent of the good since everything is from God and everything is for good. And according to our recognition of this fact and our trust in God so the underlying good will be revealed and we will merit salvation.
 Zohar 1:93b-94a
 VaYikra R. 21:4
 Sha’arei Ora 1:14a-b
 VaYikra R. 35:1
Above writing from SfasEmes blog
From Mi Yodea In Hassidic thought it has significance because it's the wrap-up of the holiday, as if to say "this represents how our chanukah went this year"; in some schools of Hassidic thought it's also seen as an extra-last-last chance for final judgment from the High Holidays (something like "the verdict is written on Rosh HaShanah and sealed on Yom Kippur ... but the book isn't closed until Hoshanah Rabba, and isn't put back on the shelf until Zos Chanukah": - at the end of Yud the Bnei Yisaschar quotes a gematria linking zos chanukah to judgement.
Chabad on Zos Chanukah