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).

Nevertheless Saul spake not any thing that day: for he thought, Something hath befallen him, he [is] not (k) clean; surely he [is] not clean.
(k) Yet he might have some business to let him.

Nevertheless, Saul spake not anything that day,.... About David's absence, took no notice of it, said nothing about it:
for he thought something had befallen him; some impurity, some nocturnal pollution, see Leviticus 15:16,
he is not clean, surely he is not clean; which he repeated in his mind for the confirmation of it, and in contempt, and to the reproach of David; and in this way he accounted for his absence the first day, and so was easy, it not being lawful and fitting for an unclean person in a ceremonial sense to eat of the peace offerings, which Saul and his family were now partaking of.

he is not clean--No notice was taken of David's absence, as he might be laboring under some ceremonial defilement.

On this (first) day Saul said nothing, sc., about David's absenting himself, "for he thought there has (something) happened to him, that he is not clean; surely (כּי) he is not clean" (vid., Leviticus 15:16.; Deuteronomy 23:11).

Something - Some accident which has rendered him unclean, and so unfit to partake of this feast, which consisted in part of the remainders of the peace - offerings, according to the law, Leviticus 7:20. Unfit also to come into any company, much more, into the king's company, lest he should pollute them also.