‘And I will pour upon the house of David.’ When is this? At some final stage, in some national blessing upon ethnic Jews in particular? No, Joel 2:28 which Peter quotes on the Day of Pentecost (Acts 2:16-17) is a parallel prophecy.
When is this? First the New Testament applies it to Christ on Calvary, and what happens to many after Calvary who come to see the real meaning of Calvary and mourn that their sins caused the Saviour of the world to have to die on a cross. Secondly, it refers to very last days when Christ comes again. John tells us in Revelation 1:7 that there will be wailing because of him when Christ appears at the second coming. Every eye will see him because those who pierced him and those responsible for his death will be raised from the dead, and they too will see him coming on the clouds of heaven. This will not be sorrow of repentance, but sorrow of regret over lost opportunity. But it does not refer to a pre-millennial return of Christ, as if only Israel only will look on him.