Exodus 12:3

In the tenth day of this month It was necessary they should now begin to prepare the passover four days before, because otherwise it would have been difficult to get ready so many lambs in Egypt, especially as they were to depart in haste; besides, this being the first instance of the celebration of the ordinance, they would require more time to prepare for a ceremony entirely new. But in future ages they did not begin the preparation till the thirteenth, the day before the passover. They shall take every man a lamb The Hebrew word signifies a lamb, or kid, (Deuteronomy 14:4,) as is evident from Exodus 12:5; for they might take either for this sacrifice: but commonly they made choice of a lamb.