8-9 On that day the Lord will protect the inhabitants of Jerusalem, so that the feeblest among them on that day shall be like David, and the house of David shall be like God, like the angel of the Lord, going before them. And on that day I will seek to destroy all the nations that come against Jerusalem. 10 “And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and pleas for mercy, so that, when they look on me, on him whom they have pierced, they shall mourn for him, as one mourns for an only child, and weep bitterly over him, as one weeps over a firstborn.

11 On that day the mourning in Jerusalem will be as great as the mourning for Hadad-rimmon in the plain of Megiddo. 12-14 The land shall mourn, each family (clan) by itself: the family of the house of David by itself, and their wives by themselves; the family of the house of Nathan by itself, and their wives by themselves; the family of the house of Levi by itself, and their wives by themselves; the family of the Shimeites by itself, and their wives by themselves; and all the families that are left, each by itself, and their wives by themselves. Zechariah 12:8-14 English Standard Version

In Chapters 12-14 of this book the prophet Zechariah uses the phrase 'on that day' some sixteen times to declare things God was going to do. The first was in vs 3 of this Chapter, when God said: “On that day I will make Jerusalem a heavy stone for all the peoples. All who lift it will surely hurt themselves. And all the nations of the earth will gather against it.” In today's verses he uses it three more times. It's a day when God will protect the inhabitants of Jerusalem, and destroy the nations that come against it. But it will also be a day of great mourning. (vs 11)

I’ve said before that it's often not easy to pinpoint the exact time or event the prophets spoke of. That was true even for the prophets themselves! The apostle Peter said: “The prophets, who long ago foretold the grace that you’ve received, searched and explored, inquiring carefully about this salvation. They wondered what the Spirit of Christ within them was saying when he bore witness beforehand about the suffering that would happen to Christ and the glory that would follow. They wondered what sort of person or what sort of time they were speaking about. It was revealed to them that in their search they were not serving themselves but you. These things, which even angels long to examine, have now been proclaimed to you by those who brought you the good news.” (1 Peter 1:10-12 Common English Bible)

So, regarding today's verses, some say they refer to the end of the ages day when Jesus Christ returns. Revelation 1:7 says: “Behold, He is coming with the clouds, and every eye will see Him, even those who pierced Him, and all tribes of the earth will wail on account of Him.” The view then is that 'on that day' refers to when God afflicts the nations who are attacking Israel.

Others, however, see this section as applying in a figurative way to when Christ first came. The apostle John tells how soldiers pierced the side of Jesus, and says it was a fulfilment of what Zechariah had said. (John 19:37) It’s the day when God sets up His kingdom, as the prophet Daniel had foretold. If this is the case, then 'the feeble who are made like David' refers to Jesus' apostles, and to Jews in Jerusalem who became Christians, starting at the Day of Pentecost. Indeed, after the apostle Peter had preached, and pointed out how they had crucified (pierced) God's Son, there was a deep mourning among the people.

The aim of these posts is not to promote one view over the other. There are books available that go deeper into the details. What I would draw from Zechariah's words is the wonderful blessing for any man or woman upon whom God pours out “a spirit of grace, and pleas for mercy, so that, when they look on Me, on Him whom they have pierced, they shall mourn for Him, as one mourns for an only child, and weep bitterly over Him, as one weeps over a firstborn.” That happened at Pentecost - but it's true also in greater or lesser degree of everyone who becomes a Christian, when we see that it was for our sins that the only begotten Son of God was wounded and pierced.