Shavuos reveals the hidden first light

וביום הביכורים.

"And on the day of the Bikkurim (first-fruits)."

The Sefas Emes begins from the Torah's name for Shavuos — Yom HaBikkurim, the day of "firsts" — and will unfold why this day is bound up with the very concept of "first."

כי יום זה הוא שעלה במחשבה מקודם כמ"ש בשביל בני ישראל שנקראו ראשית כמו כן בזמן זה עלה יום זה לפניו ובעבורו נברא העולם לכן נתעורר בו התחדשות ונק' תמיד יום הביכורים שדבוק בו הראשית לכן תקריבו מנחה חדשה.

For this day is one that arose in the [Divine] thought beforehand, as it is said, "for the sake of Bnei Yisrael who are called reishis (the first)"; likewise, in time, this day arose before Him, and for its sake the world was created. Therefore a renewal is awakened on it, and it is always called Yom HaBikkurim, for the "first" is attached to it; therefore "you shall bring a new minchah (meal-offering)."

Just as Bnei Yisrael, called "reishis," were part of the original Divine intent for which the world was created, so this day of Shavuos was "first in thought" — the purpose for which everything was made. Because the primal "beginning" clings to it, it perpetually carries the quality of renewal, which is why the Torah commands a "new" offering upon it.

והוא הטוב הגנוז שבו מתקיים העולם ובכל יום מחדש בטובו מעשה בראשית רק שהוא נסתר כמ"ש מה רב טובך אשר צפנת.

And it is the hidden good through which the world is sustained, and every day He renews in His goodness the work of creation — only that it is concealed, as it is written, "How abundant is Your goodness that You have hidden away."

This "first" quality is the ohr ha'ganuz, the hidden good that sustains the world; Hashem renews creation daily through it, but ordinarily it remains concealed, as the verse in Tehillim says of the goodness Hashem has stored away.

וביום זה נתגלה אור הצפון בפועל כמ"ש בזוה"ק שמות בפסוק ותצפנהו ג' ירחים ע"ש.

And on this day the concealed light is revealed in actuality, as it is taught in the Zohar HaKadosh, Shemos, on the verse "and she hid him for three months" — see there.

On Shavuos this normally hidden light breaks into actual revelation; the Sefas Emes points to the Zohar's discussion of Moshe being "hidden for three months," linking the theme of concealment that ultimately becomes revelation.

וכ' ולא יכלה עוד הצפינו כענין שכתוב ולא יכול יוסף להתאפק שגברו כחן של בנ"י שהקדימו נעשה לנשמע מול כן גם השי"ת עשה לפנים משורת הדין ונתגלה מה שהי' צריך להיות נסתר:

And it is written, "and she could no longer hide him," similar to what is written, "and Yosef could not restrain himself" — for the strength of Bnei Yisrael overpowered, since they placed "na'aseh" before "nishma"; correspondingly, Hashem too acted lifnim mishuras hadin (beyond the strict letter of the law), and that which was meant to remain concealed was revealed.

"She could no longer hide him" and "Yosef could not restrain himself" both express a holy force breaking through every barrier. When Bnei Yisrael declared na'aseh before nishma — committing to do before even hearing — they "overpowered," and in kind Hashem responded beyond the letter of the law, revealing the hidden light that, by ordinary measure, was supposed to stay concealed.

Summary: Shavuos, Yom HaBikkurim, is the day that "arose first in the Divine thought" — like Bnei Yisrael who are called "reishis," it is the purpose for which the world was created, and so it perpetually carries the quality of renewal. The "first" is the hidden good (ohr ha'ganuz) by which Hashem sustains and daily renews creation, normally concealed. But on Shavuos this concealed light bursts into open revelation: because Bnei Yisrael "overpowered" by placing na'aseh before nishma, Hashem responded beyond the strict letter of the law and revealed what was destined to remain hidden.