Jonathan said to David, Yahweh, the God of Israel, [be witness]: when I have sounded my father about this time tomorrow, [or] the third day, behold, if there be good toward David, shall I not then send to you, and disclose it to you?
It appears that this appeal by Jonathan to David was made during their journey together "into the field."  There was a dreadful premonition on Jonathan's part that Saul's enmity against David would terminate in Saul's being "cut off from the face of the earth."  Jonathan exacted from David a solemn oath that, "after Jonathan's death," and after David's coming to the throne, that David would remember the house of Jonathan with kindness. David honored his promise here as revealed in  2 Samuel 21:17.