[TheThe following question came up in this week's Parshat Hashavua' Chatthis week's Parshat Hashavua' Chat:]
Rashi, at the beginning of his commentary to the Torah:
אמר רבי יצחק: לא היה צריך להתחיל [את] התורה אלא (שמות יב ב) מהחודש הזה לכם, שהיא מצווה ראשונה שנצטוו [בה] ישראל, ומה טעם פתח בבראשית
Said Rabbi Isaac: It was not necessary to begin the Torah except from “This month is to you,” (Exod. 12:2) which is the first commandment that the Israelites were commanded, (for the main purpose of the Torah is its commandments, and although several commandments are found in Genesis, e.g., circumcision and the prohibition of eating the thigh sinew, they could have been included together with the other commandments). Now for what reason did He commence with “In the beginning?"
But wait... how is "החודש הזה לכם" the "first mitzva"? It is certainly not the first mitzva mentioned in the Torah, for there are three before it in Sefer Bereshis (p'ru ur'vu, mila, and gis hanashe). It is not the first mitzva following Matan Torah; that would be anochi Hashem (although some hold that that's not a mitzva, but you get the point).
So in what respect is "החודש הזה לכם" the "first commandment that the Israelites were commanded"?