John Gill Commentary 1 Samuel 20:18

"Then Jonathan said unto him, To-morrow is the new moon: and thou wilt be missed, because thy seat will be empty." — 1 Samuel 20:18 (ASV)

Then Jonathan said to David, tomorrow [is] the new moon
The first day of the month, as David had before observed, (1 Samuel 20:5) ;

and you will be missed, because your seat will be empty ;
or be inquired after, because not in his usual place at mealtime.