Albert Barnes 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, or the beginning of each month, was celebrated with special sacrifices and the blowing of trumpets (as noted in the relevant scriptures). The feast was kept with great solemnity as “a day of gladness,” and we may presume that the “peace offerings” offered on the occasion furnished the tables of those who offered.