[Q]The fourteenth day is to be remembered as observed from generation to generation for ever[QE]
Verse 14, "This day" is COMMANDED for what that very night would still happen.
"IT CAME TO PASS that at MIDNIGHT [as before commanded] the LORD smote the firstborn … Pharaoh told Moses to go … and the people took their dough and … DID ACCORDING TO THE WORD [COMMAND] and Israel JOURNEYED FROM Rameses to Succoth …
39 … they WERE THRUST OUT OF EGYPT….
41 At the END of the 430 years on the BONE-DAY it happened that ALL went OUT ... THIS IS THAT NIGHT TO BE OBSERVED IT IS A NIGHT TO BE MUCH OBSERVED UNTO THE LORD FOR HAVING BROUGHT THEM OUT.”
12:14 contains COMMAND BEFORE they would leave, TO GO feast.
“THIS—past—day, fourteenth, must be a memorial.
It has been a ‘memorial’ for Israel ever since; a memorial of the KILLING of the sacrifice; a “Feast-day”, no doubt. Till today the whole passover season of eight days is kept a memorial-feast of the PAST day “they KILLED the passover”.
“They ate the sacrifice // flesh with unleavened bread” “THIS DAY”, verses 8 and 14. they ate “in the NIGHT”.
It is only possible the past day and the following night are dated together “the fourteenth day” if it is a sunrise to sunrise day.
Egyptians reckoned days sunrise to sunrise. Israel was IN, Egypt BEFORE they had left, “the fourteenth day of the First Month its DAY first and its NIGHT last!
The next morning they began to go out.
They journeyed all day and came to halt at Succoth late next afternoon.
Then as the new day began, the 430 years were left behind
And they FEASTED THE FIRST TIME IN “THIS THAT NIGHT to be observed on “BONE-DAY”.
From that Israel was OUT, the Divine reckoning according to God’s CREATION of days, FIRST the darkness AFTERWARDS the daylight, was valid. So after Israel the previous halve-a-night and all daylight long “GOING OUT OF EGYPT”, GOD’S NEW DAY AND NEW ERA after “the end of the 430 years” were full, BEGAN, with its EVENING after sunset. Now it was the FIFTEENTH day of the First Month.
Now one may understand Exodus 12:41,42 and “the first day they ate unleavened bread”. Because it was the SECOND “first day” and the day AFTER, “the HEAD first day ye shall REMOVE LEAVEN” on the fourteenth day.
Now one may also understand 12:18, “seven days unleavened bread … from the fourteenth until the ONE and twentieth day” = 8 days!