Jamieson, Fausset and Brown on Leviticus 22:27-28

27-28. it shall be seven days under the dam--Animals were not considered perfect nor good for food till the eighth day. As sacrifices are called the bread or food of God (Le 22:25), to offer them immediately after birth, when they were unfit to be eaten, would have indicated a contempt of religion; and besides, this prohibition, as well as that contained in Le 22:28, inculcated a lesson of humanity or tenderness to the dam, as well as secured the sacrifices from all appearance of unfeeling cruelty.