Rashi: Refer to Bamidbar, 9:4:1:1.

Ramban #1: To teach us that, although in the previous Parshiyos, Moshe taught Aharon and his sons, and sometimes also to Yisrael (alias Beis-Din), ¹ the current Parshah - of Mo'adim he taught to all of Yisrael.

Ramban #2 and Moshav Zekenim (citing Megilah 32a, the Sifra and Targum Onkelos [and Targum Yonasan]): To teach us that Moshe taught Yisrael the Dinim of Pesach on Pesach, the Dinim of Shavu'os on Shavu'os and the Dinim of Sukos on Sukos. And he also taught them the secret of Ibur (the Dinim concerning adding a day on to the months and a month on to the years) that was given to him orally at Sinai.

Pesachim, 77a): To teach us that all the Korb'nos Tzibur - including the Omer and the Sh'tei ha'Lechem and their accompanying Korbanos ² override Shabbos and Tum'ah. ³