Torah is given anew each Shavuos

כמו שכתבו חז"ל שכל הנביאים קבלו נבואתם מהר סיני.

Just as Chazal wrote that all the prophets received their prophecy from Mount Sinai.

The Sefas Emes cites the teaching of Chazal (based on Shemos Rabbah and the Midrash on the nevi'im) that every navi throughout the generations drew his nevuah from the single revelation at Har Sinai — all later prophecy was already contained within Matan Torah.

כמו כן בכל שנה ושנה מקבל כל אחד מישראל בחג הזה כל מה שעתיד להבין ולחדש בתורה.

So too, in each and every year, every individual of Bnei Yisrael receives on this festival all that he is destined to understand and to innovate (l'chadesh) in Torah.

Mirroring that pattern, the Sefas Emes teaches that on Shavuos each year every Jew receives in potential the entire portion of Torah understanding and chiddushim he is meant to attain over the coming year. The whole year's Torah is granted at its source on the day of Matan Torah.

וז"ש אלמלא האי יומא כמה יוסף איכא בשוקא.

And this is what is meant by: "Were it not for this day, how many a Yosef [i.e., ordinary man] there would be in the marketplace."

He brings the Gemara (Pesachim 68b) where Rav Yosef celebrated Shavuos with a special feast, declaring that if not for "this day" — the day the Torah was given, which elevated him — he would be just another anonymous "Yosef" in the street, no different from anyone else.

האי יומא ממש.

"This day" — literally.

The Sefas Emes stresses the word "this day" in its plain, literal sense: not merely the historical Sinai, but this very calendar day, recurring each year.

זה החג שבו ניתן התורה בכל שנה:

this festival on which the Torah is given anew every year.

Shavuos is not only a commemoration of a past event; on it the Torah is actually given again each year. Rav Yosef's "this day" refers to the living, annual Matan Torah through which every Jew is lifted from being an ordinary "Yosef in the marketplace" to a recipient of fresh Torah.

Summary: Just as all the nevi'im drew their prophecy from the one revelation at Har Sinai, so each year on Shavuos every Jew receives, at its source, all the Torah understanding and chiddushim he is destined to attain in the coming year. This is why Rav Yosef cherished "this day" literally — for on Shavuos the Torah is given anew annually, raising a person from being an ordinary face in the crowd to a fresh recipient of Torah.