The new moon was a solemn and festival time, as among the Romans and other heathens, so also with the Hebrews, who solemnized it with offering peace-offerings to God, and feasting together upon the remainders of it, after the manner. See Numbers 10:10, Numbers 28:11 Psalms 81:3. I should not fail to sit with the king at meat; then he useth to expect my company above other times. Unto the third day, i.e. unto the next day but one after the new moon; as appears by comparing 1 Samuel 20:19,27,35. His meaning is not that he should hide himself in any certain place all the three days, but that he should secure himself, either at Bethlehem with his friends, or in any other place, till the third day.
Related Commentaries of 1 Samuel 20:5
1 Samuel 20:5
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.