"On the eighth day shall you give it to Me" — the Torah specifies that a first-born animal becomes eligible for the altar on the eighth day after birth. But the Mekhilta asks: is it acceptable only on the eighth day itself, or from the eighth day onward?
The answer comes through a gezeirah shavah. The word "eighth" appears here and also in (Leviticus 22:27): "From the eighth day onward it shall be accepted as an offering." In the Leviticus verse, the phrase "from the eighth day onward" makes clear that the animal remains eligible indefinitely — the eighth day is a minimum, not a maximum.
Since both passages use the word "eighth," the Mekhilta transfers the "onward" from Leviticus to Exodus. Just as there, from the eighth day on, so here, from the eighth day on. The animal can be offered on the eighth day, the ninth day, or any day thereafter.
But the transfer works in both directions. The Exodus verse establishes that the eighth day itself is the first acceptable day. This precision transfers back to Leviticus, confirming that the "eighth day" mentioned there is also the first available day — not earlier.
Two passages, each providing information the other lacks, combine through a single shared word to create a complete rule: the animal is acceptable from the eighth day onward, beginning specifically on the eighth day, with no upper limit.