Albert Barnes Commentary 1 Samuel 24:4

"And the men of David said unto him, Behold, the day of which Jehovah said unto thee, Behold, I will deliver thine enemy into thy hand, and thou shalt do to him as it shall seem good unto thee. Then David arose, and cut off the skirt of Saul`s robe privily." — 1 Samuel 24:4 (ASV)

The day the LORD spoke of... — This was how David's men interpreted divine predictions like 1 Samuel 15:28 and 1 Samuel 16:1, 12. Jonathan’s words in 1 Samuel 20:15 and 1 Samuel 23:17 show clearly that these predictions were known.