And you shall tell your son on that day, “It is because of this that God acted for me when I left Egypt” (Exodus 13:8)
Many of the mitzvos in parshas Bo relate to Pesach, remembering the Exodus and reenacting it in a fashion. Here, we are specifically instructed to tell the story of the Exodus, especially (but not exclusively) to our children. In fact, it is still a mitzvah to tell over the story of the Exodus even if one is all alone on Seder night. (In verse 3, Moshe tells the people to remember the day they left Egypt – even in the absence of others to whom to relate it.) The reason we do so much to commemorate and recall the Exodus is that it was the seminal event in the formation of the Jewish people; on that day, God took us out of Egypt to serve only Him.
This mitzvah is fulfilled at the Seder, specifically in the portion called Maggid. (The words Maggid and Haggadah come from the Hebrew word meaning to relate or to tell.) At the Seder, we drink four cups of wine, corresponding to the four terms used in the Torah to refer to our redemption, and we are obligated to explain the significance of the korban Pesach, matzah and maror (Passover offering, unleavened bread and bitter herbs). The section of the “Four Sons” uses our verse here and others to describe the ways in which different personality types might pose their questions and the proper way for us to respond to each.
The “this” in our verse refers to the matzah, maror and, in the time of the Temple, the korban Pesach. As the Haggadah discusses, this is how we know that the obligation to relate the Exodus is on the Seder night. As Rashi explains it, “Because of this – i.e., because I fulfill His mitzvos, such as this (korban) Pesach, matzah and maror.”
This mitzvah applies to both men and women in all times and places. It is discussed in Talmud Pesachim on pages 116a-b. The halachos of the Seder can be found in the Shulchan Aruch in Orach Chaim 472. This mitzvah is #157 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #24 of the 77 positive mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.