Albert Barnes Commentary Mark 2:24

"And the Pharisees said unto him, Behold, why do they on the sabbath day that which is not lawful?" — Mark 2:24 (ASV)

That which is not lawful. That is, that which they considered to be unlawful on the sabbath day. It was made lawful by Moses, without any distinction of days; but they had denied its lawfulness on the sabbath. Christ shows them, from their own law, that it was not unlawful.