"In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even" (Ex.12:18).
This was the passover meal that was eaten on the 14th day:
"And they shall eat the flesh in that night, roast with fire, and unleavened bread; and with bitter herbs they shall eat it" (Ex.12:8).
There can be doubt that the passover was on the 14th day of the month so the fist day of unleavened bread was also on the 14th:
"And in the fourteenth day of the first month is the passover of the LORD" (Num.28:16).
How to resolve the difficulty with the accepted translation of Exodus 12:18 …
It is obvious Exodus 12 covers TWO nights and TWO daylight days as happened everything that happened on those TWO days on the ONE DATE of "the fourteenth day of the First Month".
“On the fourteenth day ye shall KILL (your lamb) late in the mid-afternoon.”
“THIS day shall be unto you for a memorial; and ye shal keep it a FEAST to the LORD.”
This is the ONLY time in all Scripture that the fourteenth day is called a “Feast”. “On the fourteenth day ye shall kill the passover late in the mid-afternoon.” The KILL of the passover is called a “FEAST”. This is the first, the last and the only instance a Scripture states the kill of the passover on the fourteenth day, a FEAST.
“And they shall EAT the flesh in THAT NIGHT WITH unleavened bread”—before the “morning” because next “morning, ye shall let nothing of it remain.” Verse 14, “This day”—“day”-time of the “kill” then “night”-time of the “feast” [eat]—“ye shall keep a feast.”
“Seven days shall there be no leaven found in your houses”.
“Seven days shall ye eat unleavened bread [as follows]:
The very first day [the fourteenth day] ye shall PUT AWAY leaven out of your houses.
For whosoever eateth leavened bread from the first day until the seventh day, that soul must be cut off from Israel.”
“Ye shall observe unleavened bread in this selfsame Bone-day I have brought your armies out … Therefore shall ye observe THIS day …
“… on the fourteenth day of the month late, mid-afternoon [‘ereb’].”
Here the pericope STOPS.
A NEW pericope starts,
“Ye shall eat unleavened bread until the one and twentieth day of the month late after sunrise in the night”—‘ereb’.
Note translation of ‘ereb’ with the meaning “late in the mid-afternoon” for the KILL;
but with the meaning “late after sunrise in the night” before the “morning”, verse 10, for the “FEAST” or “EAT” of unleavened bread.
Ever after Exodus 12:14 and 18 the two days and two nights originally dated “fourteenth”, became dated two days, “fourteenth” and "fifteenth of the First Month". Therefore is it illegitimate to only mention “Num.28:16” but not verse 17 also!