Albert Barnes Commentary Matthew 12:2

"But the Pharisees, when they saw it, said unto him, Behold, thy disciples do that which it is not lawful to do upon the sabbath." — Matthew 12:2 (ASV)

Upon the sabbath day. The Pharisees, doubtless desirous of finding fault with Christ, said that, in plucking the grain on the sabbath day, they had violated the commandment. Moses had commanded the Hebrews to abstain from all servile work on the sabbath (Exodus 20:10; Exodus 35:2–3; Numbers 15:32–36).

On any other day this would have been clearly lawful, for it was permitted (Deuteronomy 23:25).