Exodus 12:6 It shall be yours for examination until the fourteenth day of this month; the entire congregation of the assembly of Israel shall slaughter it in the afternoon.
Ok. Simple. The lamb is slaughtered on the 14th in the afternoon.
Exodus 12:8 They shall eat the flesh on that night - roasted over the fire - and unleavened bread; with bitter herbs shall they eat it.
When it's the 14th, and then the sun sets, it becomes the 15th. The Passover meal... the feast of unleavened bread... is eaten on the 15th.
Why in the hell did anybody start talking about the 13th? Exodus is crystal clear.
The 13th isn't the issue here, the 14th is the issue.
So let's look at more of that passage from Exodus.
"On the tenth day of this month (
Ahib in Canaan,
Nisan in Babylon) each man is to take a lamb for his family, one for each household. . .Take care of them until the
fourteenth day of the month,
when all the people must
slaughter them at
twilight
[literally, "between the two evenings", which could mean
either
1) between the decline of the sun and sunset (Nisan 13), or
2) between sunset and nightfall (Nisan 14)]
That
same night (Nisan 14) they are to
eat the meat roasted over the fire." (Ex 12:3-8)
The testimony of the Church since its first-century beginning is that the meaning was
2), above, and that
the Lamb was slaughered, roasted and eaten on Nissan 14, which is the date of Passover.
Pegg (posts
#20 and
#102) and
Onlooker (post
#86, #112) have it exactly right.
See post #55 for more information.