All of our Yomim Tovim are celebrated to commemorate an important incident that happened in our rich Jewish history. Every Yom Tov, and Shabbos too is established to celebrate and remember something.
On Pesach we remember our redemption by eating Matza, Maror, drinking four cups of wine, and telling the story of our redemption. On Succos we remember the Clouds that protected the Jews in the desert, by moving out of our homes and into huts. Why is there no special mitzvah to remember the events of Shavuos? After all, it is the day we received the Torah – and our identity.
Rav Ahron Kotler answers that a remembrance reminds of something that one can forget, and therefore needs external reinforcement. However, the Yom Tov of Shavuos wasn’t a one-off event; the giving of the Torah is constant, as it says בכל יום יהיו בעיניך כחדשים – each day, it should be as if the Torah is new to you. There is nothing to commemorate, if we are in the middle of the event itself!
Here’s a story to illustrate this point. In the town of Dvinsk, resided Rav Meir Simcha, and the Rogatchover Gaon. Their respect for each other was exemplary, even though each one was a rav of a different community. Once, Rav Meir Simcha wanted to compliment his peer, who was universally acclaimed as the genius of that generation. “People say that the Rogatchever has a photographic memory”, announced Rav Meir Simcha, “I say, not at all. How could we consider someone who reviews the entire Torah every day with having a photographic memory? It’s not down to memory at all, he just learned it that day!”