Charles Spurgeon Commentary Zechariah 12:10

"And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplication; and they shall look unto me whom they have pierced; and they shall mourn for him, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his first-born." — Zechariah 12:10 (ASV)

And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplications:

This is a promise concerning Israel. Long have the Jews rejected the Christ, but the day is coming when they shall acknowledge Jesus of Nazareth to be the promised Messiah. In that day, this promise will be fulfilled. God must always give "the spirit of grace" before men will pray properly; and wherever grace is given, there is always true prayer.

And they shall look upon me whom they have pierced, and they shall mourn for him, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his firstborn.

Discovering that they have rejected the true Messiah, they will be overcome with the most acute grief that was ever endured, grief altogether inconceivable.