The Torah tells us the story of the first Passover holiday that was celebrated at the time that the Jewish people were leaving Egypt (Shemot 12) and specifically commands to continue celebrating the anniversary of this event when they arrive in the Promised Land (12:25). Furthermore, the Torah emphasizes that this commemoration is an obligation for all time (12:24). Yet we know that the rules and regulations that applied to the Children of Israel at the time that they were leaving Egypt are different than those that future generations were obligated to follow. Some of the differences are clearly stated in the Torah itself. For example, the Jewish people in Egypt were told to prepare the korban Pesah – the Passover sacrifice – beginning on the tenth day of the month of Nissan (12:3), and that the blood of the sacrifice was to be saved in order to use it for painting the doorposts of their houses (12:7; 22-23). These mitzvot applied only that first year, and not in subsequent celebrations of the holiday.
Clearly, many of the rules apply whenever Jews celebrate Pesah, whether in Egypt, Israel or in the Diaspora. Yet, the pesukim in Sefer Shemot are not clear with regard to establishing which rules were meant to apply for all generations and which only for the first Passover.
As basic a rule as the law forbidding eating hametz is subject to a disagreement in the Gemara. Rabbi Yosei HaGelili is quoted in a baraita as saying that during the Exodus the prohibition to eat hametz lasted only one day. While this does not have practical ramifications for us today, when it is clear that the prohibition is in place for the entire seven day holiday, it points to the different possible interpretations of the passages in Sefer Shemot. Rabbi Yosei HaGelili interprets the juxtaposition of the statement “…and you shall not eat hametz” to “today you are leaving” (see Shemot 13:3-4) as indicating that the prohibition of hametz during yetziat mitzra’im (the exodus from Egypt) was limited to that day only.