The day here spoken of, is the day of Jerusalem's defence and deliverance, that glorious day when God will appear for the salvation of his people. In Christ's first coming he bruised the serpent's head, and broke all the powers of darkness that fought against God's kingdom among men.
In his second coming he will complete their destruction, when he shall put down all opposing rule, principality, and power; and death itself shall be swallowed up in that victory. The Holy Spirit is gracious and merciful, and is the Author of all grace or holiness.
He, also, is the Spirit of supplications, and shows men their ignorance, want, guilt, misery, and danger. At the time here foretold, the Jews will know who the crucified Jesus was; then they shall look by faith to him, and mourn with the deepest sorrow, not only in public, but in private, even each one separately.
There is a holy mourning, the effect of the pouring out of the Spirit; a mourning for sin, which quickens faith in Christ, and qualifies for joy in God. This mourning is a fruit of the Spirit of grace, a proof of a work of grace in the soul, and of the Spirit of supplications.
It is fulfilled in all who sorrow for sin after a godly sort; they look to Christ crucified, and mourn for him. Looking by faith upon the cross of Christ will cause us to mourn for sin after a godly sort.