Why the plural, "pesachim"? There are several components to Pesach, including the time before Pesach that is necessary for preparation. And so we start with the biblical verses that establish those rules of no leaven in one's home on Pesach. And the analysis of these pages on the opening mishnah is dramatically different from what we've spent real time with before. Namely, word play on how the discussion of checking the home for chametz (leavening) pre-Pesach begins. [What's What: Midrash Halakhah] The Gemara wants to know why this tractate begins with the word that means light, though here, it seems to mean night, or evening, especially given that everyone knows when we do Bedikat Chametz. Of course, the midrash halakhah of this daf explores the verses and tannaitic literature that allows for this word to mean evening.