Charles Ellicott Commentary 1 Samuel 20:29

"and he said, Let me go, I pray thee; for our family hath a sacrifice in the city; and my brother, he hath commanded me [to be there]: and now, if I have found favor in thine eyes, let me get away, I pray thee, and see my brethren. Therefore he is not come unto the king`s table." — 1 Samuel 20:29 (ASV)

Our family hath a sacrifice in the city; and my brother, he hath commanded me. —Jonathan answers the king’s question in the way previously agreed upon between him and David. He quotes the excuse in David’s own words.

The Septuagint, instead of “my brother,” has “my brothers.” It thus alters the original, not understanding the singular “brother,” Jesse, their father, being still alive. The brothers collectively might, the Septuagint seemed to think, have invited David to the family sacrificial feast. Dean Payne Smith suggests that as the ceremony was not a private family gathering, but one shared in by the district, the “brother” (probably the eldest) was quite likely the convener of the absent member of the house of Jesse.