Psalm 78 (Psalm 77 in the Douay-Rheims numbering) opens with the teacher's manifesto: 'I will OPEN MY MOUTH IN PARABLES: I will utter propositions [hidden things] from the beginning' — and then retells all Israel's history as one long lesson.
Matthew places the fulfillment on the day of the great parables — the sower, the wheat and the tares, the mustard seed, the leaven — taught to crowds so large that Jesus sat in a boat just off the shore: 'All these things Jesus spoke in parables to the multitudes: and without parables he did not speak to them. That it might be fulfilled which was spoken by the prophet, saying: I will open my mouth in parables, I will utter things hidden from the foundation of the world' (Matthew 13:34–35). The natural amphitheatre of the 'Cove of the Sower' near Capernaum, where the land curves around the water, is the traditional setting.
The parable itself was the fulfillment's form: the hidden things of the Kingdom given in stories a child could carry and a scholar could drown in — 'he that hath ears to hear, let him hear.'