Let us go to Luke 22:7. Remember, what we are looking at here is a tradition of those people. It is not something that God required, but it was a tradition of those people. It was the first day of unleaven. Not "Unleavened Bread," but simply the first day that they were to be unleavened.
What Luke adds here is the definite article "the." "Then came the day of the unleavens." I am going to paraphrase this for you. "And came the day of the unleavens in which it was obligatory for the Passover to be killed."
I inserted the word "obligatory" because the Greek there is very strong. It was mandatory. It implies under the compulsion of law. It is telling us that the killing of the Passover lamb was required by compulsion of God's law to be killed at a certain time, and that certain time was ben ha arbayim—between the two evenings—at the beginning of the 14th.
Also he is telling us that "the day of unleavens" included part of the 14th, because preparations for Unleavened Bread continued after the 14th had arrived.
