Why will the nation of Israel suddenly mourn when they see Jesus?
Israel will mourn because, as Zechariah foretold, they will look on the One they pierced and suddenly recognize who Jesus truly is, just as Paul understood Him on the Damascus road. That national awakening will produce deep grief over their past rejection, and Isaiah says the nation will be born in a day.
Les teaches that Paul likens himself to the whole nation of Israel. Paul was born as a preemie, and at some future date the whole nation of Israel will experience the same kind of salvation Paul did on the road to Damascus. Zechariah says that all of a sudden they will look on Him whom they pierced and mourn for Him as one mourns for an only child. The reason is that the nation will suddenly understand who Jesus really is, just as Paul saw on the Damascus road. Isaiah also says the nation will be born in a day, Isaiah 66. Although Israel is set aside and blind while God deals with the Gentiles, Israel is still in God's program and will yet experience her greatest glory.
And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplications: 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.