ISB ~ Matthew 12 v2

not lawful … on the sabbath:The Pharisees charge the disciples with violating Ex 34:21, which forbids harvesting on the Sabbath. Although Deut 23:25 differentiates between plucking grain and harvesting it, the Pharisees forbade even plucking grain by a rigid extension of the Exodus prohibition.

• Allegorically (St. Hilary, In Matt. 12, 2): Christ’s passing through the field signifies his passing into the world through the Incarnation. The standing grain is the harvest of souls ready to believe in the gospel and be gathered into the Church by the hungry disciples.