(Zechariah 12:9-10 NIV) On that day I will set out to destroy all the nations that attack Jerusalem. {10} "And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, and they will mourn for him as one mourns for an only child, and grieve bitterly for him as one grieves for a firstborn son.

How and when did they pierce God? And why would they mourn? Isn't because it was the Jews that crucified Jesus? And they now recognise him as God?