Zechariah 12:10
"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 min, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his firstborn."

Messianic Biblical position as written by Shalach Ministries:

I will pour  Proverbs 1:23,; Isaiah 32:15,; Isaiah 44:3,4; Isaiah 59:19-21; Ezekiel 39:29,; Joel 2:28,29; Acts 2:17,33; Acts 10:45; Acts 11:15;  Titus 3:5,6 

the house of David  Zechariah 12:7 

the spirit  Psalm 51:12 

of supplications  Jeremiah 31:9;  Jeremiah 50:4; Romans 8:15,26; Ephesians 6:18; Jude 1:20

they shall look  This clearly relates to the crucifixion of Jesus of Nazareth, and to His being pierced by the Roman soldier’s spear.  We have the authority of the inspired apostle John for affirming this position.  This application also agrees with the opinion of some of the ancient Jews, who interpret it as the Messiah, the son of David.  Jarchi and Abarbanel refer it as the death of the Messiah, the son of Joseph, whom they say was to be the suffering Messiah, while the former is to be the triumphant Messiah. Psalm 22:16,17; John 1:29; John 19:34-37; Hebrews 12:2; Revelation 1:7 

they shall mourn  Jeremiah 6:26; Amos 8:10; Matthew 24:30; Matthew 26:75; Acts 2:37; II Corinthians 7:9-11

Israel receives the Spirit of grace, and grieves for the One whom they have pierced:

1. Mourning for the pierced One, "Yeshua":

         a. It all begins with an outpouring of the Holy Spirit; bringing grace and repentant prayer to the Jews.
         b. Notice who the pierced One is: Yahweh Himself, showing powerfully that "Jesus is Yahweh".

       i. Christ was pierced: His head with thorns, hands and feet with nails, and His side by the Roman spear.
      ii. Paul catches some of this mystery when He insists that the Jews had "crucified the Lord of glory" (I Corinthians 2:8).
          c. Firstborn was synonymous with the most beloved; his loss would most deeply wound a family.
          d. Early translators had a very hard time with this verse, and tried to change it. After all, how could the Lord Himself be "pierced"--that is, murdered? But Jesus fulfills it perfectly.