When do they come to see Hashem's countenance?
Radak, Malbim: On the three festivals.
Why does it say "Remos Chatzerai"?
Rashi: Why do you trample My Chatzeros, since your hearts are not complete with Me?
Radak: You come in front of Me and rebel against My word. This is not honor, rather, disgrace, you trample My Chatzeros. Yisraelim would stand in the Chatzeros; only Kohanim enter the Heichal.
Malbim: A third kind of Korbanos are Nedavos that Hashem made obligatory, i.e. when they enter the Mikdash each festival, they must bring Olas Re'iyah and Shalmei Chagigah. The purpose is to ascend to Ir ha'Kodesh three times a year, and learn from Yir'as Hashem and His ways from the Kohanim and Nevi'im - "Lema'an Tilmad Leyir'ah Es Hashem Elokecha" (Devarim 14:23). However, you do not intend to learn Yir'as Hashem; there is no Mitzvah. Therefore, you trample My Chatzeros for naught.
