Exodus 12:3 - Albert Barnes' Notes on the Bible

A lamb - The Hebrew word is general, meaning either a sheep or a goat - male or female - and of any age; the age and sex are therefore epecially defined in the following verse. The direction to select the lamb on the tenth day, the fourth day before it was offered, was intended to secure due care in the preparation for the great national festival. The custom certainly fell into desuetude at a later period, but probably not before the destruction of the temple.