Jamieson, Fausset and Brown on 1 Samuel 20:5

5. David said unto Jonathan, Behold, to-morrow the new moon, and I should not fail to sit with the king at meat--The beginning of a new month or moon was always celebrated by special sacrifices, followed by feasting, at which the head of a family expected all its members to be present. David, both as the king's son-in-law and a distinguished courtier, dined on such occasions at the royal table, and from its being generally known that David had returned to Gibeah, his presence in the palace would be naturally expected. This occasion was chosen by the two friends for testing the king's state of feeling. As a suitable pretext for David's absence, it was arranged that he should visit his family at Beth-lehem, and thus create an opportunity of ascertaining how his non-appearance would be viewed. The time and place were fixed for Jonathan reporting to David; but as circumstances might render another interview unsafe, it was deemed expedient to communicate by a concerted signal.

1Sa 20:11-23. Their Covenant Renewed by Oath.