Charles Ellicott Commentary 1 Samuel 20:5

"And David said unto Jonathan, Behold, to-morrow is the new moon, and I should not fail to sit with the king at meat: but let me go, that I may hide myself in the field unto the third day at even." — 1 Samuel 20:5 (ASV)

The new moon. — Regarding the religious ceremonies connected with the day of the new moon at the beginning of each month, see the Mosaic enactments in Numbers 10:10; Numbers 28:11–15.

At the court of Saul, the feast seems to have been carefully observed, doubtless with the blast of trumpets and with solemn burnt offerings and sin offerings, for we notice in this narrative that the plea of possible ceremonial uncleanness was immediately accepted as an excuse for absence (see 1 Samuel 20:26).

The sacrificial and ceremonial rites were accompanied by a state and family banquet, at which David, as the king’s son-in-law and also as holding a high position in the royal army, was expected to be present.

Jonathan persisted in viewing his father’s later designs against David’s life as simply frenzied acts, resulting from his distressing illness. He evidently believed that after his father's strange seizure at Ramah, Saul would return and treat David with the confidence of former days when they met at the feast of the new moon. David, however, believed otherwise and was convinced, to use his own expressive words, that there was but a step between him and death. Therefore, he would not entrust himself to Saul’s hands until his friend had carried out the experiment he suggested.