Pesachim פסחים

It begins with the search for leaven by candlelight on the eve of the fourteenth of Nisan, and goes on: what counts as leaven, what to do with what is found, how a house is cleared. The middle of the tractate is the offering itself: how it was brought in the Temple in three shifts, how the Hallel was sung while it was slaughtered.

The last chapter describes the evening of the seder: the four cups, the order, the son's questions that begin "why is this night different", and the rule that the telling goes "from disgrace to praise". Almost everything said at the table on Passover night already stands here.

Order: Moed. There is Gemara in both the Babylonian and the Jerusalem Talmud.