Two Passover services were apparent in the Old and New Testaments.
The first Passover was a commemoration of the initial killing of the
Passover lamb in Egypt. This lamb was killed after sunset by the
individual households, who then painted its blood on the doorposts
and lintels of their homes. This Passover was observed on the 13th
night, following the daylight hours of the 13th day.
The second Passover was a thanksgiving ceremony held at the place where
God chose to place His name. The animals eaten were firstborns taken
from the flock and from cattle and oxen. The animals were slaughtered
and boiled before sunset in preparation for the start of this Passover
service. The service was observed from sunset to midnight on the 14th,
immediately prior to the start of the 15th day. To participate in this
ceremony one had to be ritually clean and circumcised.
This 14th Passover initiated a period of seven nights of feasting,
ie 14th to 20th, which all commemorated the journeying of the children
of Israel from Egypt.
Both the 13th and 14th Passovers required all the meat to be eaten that
night. The meat was to be eaten with unleavened bread and bitter herbs
and none of the bones of the Passover animals were to be broken.
Due to the confusion which has surrounded the Passover observances many
translations have in error added the word 'lamb' (or 'lambs') in verses
where it is inappropriate.