Be Ready for Three Days and the Laws of Purity at Sinai

"And he said to the people, 'Be ready for three days'" (Exodus 19:15). One might think Moses said this thing on his own authority. Scripture says, "And let them be ready for the third day" (verse 11), and here "Be ready for three days." "Be ready" and "be ready" form a verbal analogy [gezeira shava]: just as the "and let them be ready" stated there is something holy that the LORD told Moses, so too the "be ready" stated here is something holy that the LORD told Moses. Rabbi says: Its place is decided on its own [it needs no inference]. "And sanctify them today" so that they should immerse on the fourth day. Rabbi Elazar ben Azariah says: How do we know that a woman who emits semen on the third day is impure? As it says, "Be ready for three days; do not draw near to a woman." Rabbi Ishmael says: Sometimes these are four periods, sometimes they are five, sometimes they are six. Rabbi Akiva says: They are always five.