Malbim: There are four reasons. (a) I commanded your fathers. Children must guard Mitzvos that their fathers accepted on themselves and on their seed after them. (b) He commanded them when he took them out of Egypt. He took them on condition that they cease being Egypt's slaves, and become His servants. (c) I stipulated that you heed My voice and be My Am. If you abandon the Bris, you are not My Am, and I am not your G-d. (d) Refer to 11:5:1:1.

Radak: It was at the time that He took us out; this Bris was [at Matan Torah,] 50 days after Yetzi'as Mitzrayim. "Yom" is in place of Es or Zman, like "v'Hayah ba'Yom ha'Hu Shoresh Yishai" (Yeshayah 11:10), "v'Charah Api Vo va'Yom ha'Hu" (Devarim 31:17).

Radak: They were in the smelter's pot, just like they refine gold and silver in a pot.

Radak: The Mitzvos. "Shim'u v'Koli" - the Mitzvos that He will command them.