He is not clean - The new moon being a religious feast, and the meat to be eaten being peace-offerings, no one could assist at the feast who had any ceremonial uncleanness upon him (marginal references).
Related Commentaries of 1 Samuel 20:26
1 Samuel 20:26
26 Nevertheless Saul spake not any thing that day: for he thought, Something hath befallen him, he is not clean; surely he is not clean.