Rabbi Acha bar Rabbi Oshiyah laid out the precise timeline of the first Passover. God spoke to Moses on the first of the month (Rosh Chodesh). The Israelites selected their lambs on the tenth. And the slaughtering took place on the fourteenth.
But could the timeline be read differently? Perhaps God spoke and the Israelites selected their lambs both on the tenth, with only the slaughtering on the fourteenth. The Mekhilta considers this alternative and rejects it based on a close reading of the text.
The key is the word "saying" at the end of (Exodus 12:3): "Speak to the whole congregation of Israel, saying." If the speaking and the selection happened on the same day, why would Scripture add the seemingly redundant word "saying"? The extra word serves to create a separation — a gap in time between the act of speaking and the act of selecting the lamb. The word "saying" drives a wedge between two events that might otherwise be collapsed into one.
This kind of analysis is characteristic of the Mekhilta's approach to Scripture. Every word matters. Every apparent redundancy carries legal weight. The rabbis treated the Torah like a precisely engineered document where nothing is wasted. A single "extra" word could determine the ritual calendar of an entire nation. Rabbi Acha's reading established the authoritative three-stage timeline: divine speech on the first, selection on the tenth, slaughter on the fourteenth.