John Gill 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)

And he said, let me go, I pray you, for our family has a
sacrifice in the city
In the city of Bethlehem where they lived, a peace offering on account of the new moon, and as an anniversary thanksgiving for the mercies of the year, (1 Samuel 20:6) ; and seeing, though he was not at the feast in one place, he observed it in another, his absence at court was the more excusable; and the rather, since it was kept by him with his own family, in his own city: besides, it is added,

and my brother he has commanded me [to be there] ;
his elder brother Eliab, whose commands, as a younger brother, he judged he ought to obey; it is probable his father was now dead, since no mention is made of him, and his elder brother took upon him the command of the family:

and now, if I have found favour in your eyes, let me get away, I pray
you, see my brethren :
he should take it as a favour to have leave to depart, and be free for the present from waiting upon the king at court, and so have an opportunity of seeing his brethren, for which he had a great desire; having not seen them a long time, not even since he saw them in the camp, when he slew Goliath:

therefore he comes not to the king's table :
this was the reason of it, at least one reason of it, and Jonathan was not obliged to tell the whole.