Then Jonathan said to him, Tomorrow is the new moon: and you will be missed, because your seat will be empty.
"On the third day"  (1 Samuel 20:19).  This refers to the third day of the feast.  The first day of the feast, David would be missed; but the real test would come if he missed the second day of the feast.  The feast was apparently a night affair, because it was "in the morning"  (1 Samuel 20:35) of that third day (following the second of the feast) that Jonathan would give the pre-arranged signal to David.