Passover in The Old Testament
nGod commanded all believers (Israelites and strangers) to keep the passover, which was to be observed on 14th day of the first month (Abib/Nisan) of the biblical Hebrew calendar.  nThey were to select a perfect lamb without blemish and then it was to be sacrificed.  nThey celebrated this as a memorial of what God did to save them from the plagues and bondage in Egypt.