Every Shabbat is Called Shabbat Hagadol
From the Kol Emunim newsletter by the Shomrei Emunim Rebbe, p. 2, bottom-left:
In the blessing of Retzei that we say in Birkat Hamazon on Shabbat day, we ask, "ובמצות יום השביעי השבת הגדול והקדוש הזה...And with the mitzvah of the seventh day - this great and holy Shabbat". We need to understand what it means by השבת הגדול - the great Shabbat.
We can possibly explain this based on how we find that the Shabbat before Pesah is called Shabbat Hagadol as is explained in the Tur O"H Siman 430. The holy books explain the reason for this: since during the following week there was the Exodus, and since all blessings are dependant on Shabbat as is mentioned in the Zohar (II:88b), it comes out that the Exodus stemmed from the preceding Shabbat. And that is why it is called Shabbat Hagadol, based on the Yad Hagedola - the great hand - which Hashem placed against Egypt during the time of the Exodus.
And according to this, it appears that also the Shabbat before the Final Redemption will be called Shabbat Hagadol since that will start the flowering of our Redemption and the freedom of our souls. This is why we say every Shabbat השבת הגדול והקדוש הזה - this great and holy Shabbat - because every Shabbat we hope and pray that our salvation is close in coming any day. It comes out that this (הזה) Shabbat is a Shabbat Hagadol. Once could also say that that which Haza"l say (Shabbat 118b) that if the Jewish people would only keep 2 Shabbatot, we would immediately be redeemed. Therefore, we say in Birkat Hamazon, "This great and holy day" because we hope that perhaps it will be this Shabbat that we will keep properly and we will merit through this to the True Redemption speedily in our days, Amen.
And we can possibly also say that by the Exodus, it was just called Shabbat Hagadol, but in the future, it will be called Shabbat Hagadol Vehakadosh. This is because it will be called "Hagadol" similar to the Exodus, as it says (Micha 7:15) כימי צאתך מארץ מצרים אראנו נפלאות - like the days of your leaving Egypt, I will show him wonders. It will be called "Hakadosh" because of the verse (Zecharia 14:3)וְיָצָא יְהוָה, וְנִלְחַם בַּגּוֹיִם הָהֵם, כְּיוֹם הִלָּחֲמוֹ, בְּיוֹם קְרָב Then shall the LORD go forth, and fight against those nations, as when He fighteth in the day of battle. And the early sages have said that the Gog Umagog War will be felt on Shabbat day. Our rabbis have given a hint to this when they said, "Whowever keeps 3 meals on Shabbat is saved from 3 things, one of which is Gog Umagog. And the prophet already said (Yehezkel 38:23) "וְהִתְגַּדִּלְתִּי, וְהִתְקַדִּשְׁתִּי, וְנוֹדַעְתִּי, לְעֵינֵי גּוֹיִם רַבִּים Thus will I magnify Myself, and sanctify Myself, and I will make Myself known in the eyes of many nations". And this is our request and our expectation - that on this Shabbat day, Hashem's great name will be magnified and sanctified via Am Yisrael's victory in the Gog Umagog War, and this Shabbat will thereby be a great and holy Shabbat - speedily in our days, Amen.