This tractate investigates all the laws of Passover. This most significant spring-time holyday (week, really) has several essential elements including abstaining from any leaven, eating of matzah, holding a seder and pilgrimage and pascal sacrifice (in the days when the Temple still stood).
It begins, though, with a discussion of the night before Passover begins. The Holiday begins at sunset on the fifteenth of Nisan. The Mishnah begins:
ON THE LIGHT [OR] OF THE FOURTEENTH [OF NISAN] A SEARCH IS MADE FOR LEAVEN BY THE LIGHT [OR]OF A LAMP
The Rabbis discuss at some length the meaning of the Hebrew word “or” in this context. Usually translated as “light” it is an odd word to use in this context because it is an unclear time. Does it mean when light begins or when light ends? Daybreak or Sunset? Rather important to know!
Many different texts are marshaled to show what “or” means in context, in effect proving both sides. Ultimately it is shown that in this situation “or” means “evening” - that is as the light is leaving. It is proven by the case of bonfires which were lit to announce the new moon:
And when were the bonfires lit? On the evening [‘or’] after [the intercalated day]. This proves that ‘or’ is evening. This proves it.
We search for leaven after sunset, 24 hours before Passover begins. Now we know when. How about where and what? Stay tuned!