Joy of Sukkos endures even in galus

שמחת בית השואבה שמשם שואבין רוח הקודש.

The Simchas Bais HaShoeivah (the Rejoicing of the Water-Drawing), for from there they would draw ruach hakodesh (the spirit of holiness).

The festive water-drawing celebration of Sukkos was a wellspring; from its joy the people would actually "draw up" ruach hakodesh.

פי' משם מהשמחה של הימים אלו.

That is: "from there" means from the simchah (joy) of these days.

The Sefas Emes clarifies that the source from which ruach hakodesh is drawn is the very joy of these days of Sukkos.

ואף גם עתה שאין לנו לקיים זאת השמחה כראוי אעפ"כ הוא הבטחה והיית אך שמח.

And even now, when we are unable to fulfill this simchah properly, nevertheless it is a promise: "and you shall be only joyful."

Even in our times, without the Bais HaMikdash and its full celebration, the Torah's words "and you shall be only joyful" stand as a guarantee that the joy is still attainable.

אך חלק פי' אף בדורות השפלים שלא תהי' השמחה בשלימות.

The word "ach" ("only/but") is limiting (it "divides") — meaning that even in the lowly generations, when the simchah will not be complete, [the promise still holds].

Chazal teach that the word "ach" always restricts; here the Sefas Emes reads it as including even the diminished generations, when our joy is only partial — yet the assurance of joy applies to them too.

וכן איתא במד' על פסוק אך בחמשה עשר לחודש כו' מאי אך כו' ע"ש בילקוט.

So too it is brought in the Midrash on the verse "ach (but/only) on the fifteenth of the month…" — what does "ach" come to teach? — see there in the Yalkut.

He cites a parallel Midrash that also probes the limiting word "ach" in connection with the fifteenth of the month (Sukkos), supporting his reading.

ואפשר ז"ש חז"ל לרבות לילי יו"ט האחרון היינו הימים טובים בגלות הדומה ללילה באחרית הימים כנ"ל:

And it is possible that this is what Chazal meant by "to include the night of the last Yom Tov" — namely the Yamim Tovim in galus, which resembles night, at the end of days, as above.

The Sefas Emes suggests that Chazal's ruling to "include the last night of Yom Tov" alludes on a deeper level to the festivals as kept during galus — a time as dark as night — in the final generations, which are likewise embraced by the promise of joy.

Summary: The Simchas Bais HaShoeivah was a source of ruach hakodesh drawn from the joy of Sukkos. Reading the limiting word "ach," the Sefas Emes finds a promise — "you shall be only joyful" — that reaches even the lowly generations and the festivals kept in the darkness of galus, where the joy of Yom Tov, though incomplete, still endures.