1 Samuel 13:8 - Matthew Poole's English Annotations on the Holy Bible

Seven days; not seven complete days; for that the last day was not finished plainly appears from Samuel's reproof, which had then been groundless and absurd, and he had falsely charged Saul with breaking God's command therein, 1 Samuel 13:13. And as Samuel came on the seventh day, and that with intent to sacrifice; so doubtless he came in due time for that work, which was to be done before sun-setting, Exodus 29:38,39. So Saul waited only six complete days, and part of the seventh, which is here called seven days; for the word day is oft used for a part of the day, as among lawyers, so also in sacred Scripture; as Matthew 12:40, where Christ is said to be in the heart of the earth three days and three nights, i.e. one whole day, and part of the other two days. Moreover this place may be thus rendered: He tarried until the seventh day, (as this same phrase is used, Genesis 7:10, Heb. until the seventh of the days,) (as the Hebrew lamed is oft taken,) the set time that Samuel had appointed.