We are able to mourn because we have come to terms with our sins, have repented and been baptized.
Why do we mourn? We mourn because the Ruach (Holy Spirit) convicts us of our sins and we recognize the sins of the world and how it grieves our messiah Yeshua because we crucify him all over again and subject Him to open shame (Hebrews 6:6). Yeshua gave us the following promise about the Ruach Hakodesh in John 16:8 “When he comes, he will prove the world to be in the wrong about sin and righteousness and judgment.” Those are the three-fold ministry of the Ruach according to Yohanan (John). So, based on this conviction we will mourn and seek the face of the Lord concerning His impending judgement of the church first, and a sinful world second. Because now our eyes have been opened by the Spirit of God and we can understand the signs of the times and the judgement of God. Yeshua says those who mourn are the ones who will be comforted because they grieve the sins and have separated themselves from it.
Joel 2:17 tells us “Let the priests, the ministers of the LORD, weep between the porch and the altar, and let them say, spare thy people, O LORD, and give not thine heritage to reproach, that the heathen should rule over them: wherefore should they say among the people, where is their God?” Who is the priest of God? Peter tells us that we are all a holy priesthood and a holy nation unto God! So, God expects His children who understand the times to weep. And why are they weeping between the porch and alter (this is the alter of incense in the Holy Place)? It is because our Messiah Yeshua and Great High Priest has moved into the Most Holy Place (Holy of Holies), He has entered the second Veil, in the Heavenly Sanctuary and performing the Work of Judgement. We have to remember we are the People of the Judgement (the church of Laodicea) So we must be standing in the Holy Place in the spirit to plead and intercede and asking God to spare us in His Judgement and have mercy! The book of Hebrews 8 & 9 explains the Priesthood of Yeshua, “A minister of the sanctuary, and of the true tabernacle, which the Lord pitched, and not man” (Hebrews 8:2). He has entered the Holy of Holies as is done in YOM KIPPUR, to offer the blood He shared on Calvary once for our sins. “Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us.” Hebrews 9:12 “For Christ is not entered into the holy places made with hands, which are the figures of the true; but into heaven itself, now to appear in the presence of God for us:” Hebrews 9:24 This must be the solemn and holiest period for us all who believe and understand, just as we celebrate day of Atonement every year.
In Isaiah 61:1, Yeshua states the following “The Spirit of the Lord God is upon me; because the Lord hath anointed me to preach good tidings unto the meek; he hath sent me to bind up the broken-hearted, to proclaim liberty to the captives, and the opening of the prison to them that are bound;2 To proclaim the acceptable year of the Lord,” Yeshua ended the statement here in His first advent but now in His second advent, the remainder of the verse 2 applies…” and the day of vengeance of our God; to comfort all that mourn;3 To appoint unto them that mourn in Zion, to give unto them beauty for ashes, the oil of joy for mourning, the garment of praise for the spirit of heaviness; that they might be called trees of righteousness, the planting of the Lord, that he might be glorified.” That is why we have to mourn because the vengeance of God will spare those who mourn and have separated themselves from sin, they will receive beauty for ashes.
In Ezekiel 9, Before the Babylonian captivity of Judah, according to Ezekiel, God sent His angels to mark the foreheads of all those who mourned and sighed because of the Sins of Jerusalem. And the rest were executed. Ezekiel 9:4 “And the Lord said unto him, go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that be done in the midst thereof. 6 Slay utterly old and young, both maids, and little children, and women: but come not near any man upon whom is the mark; and begin at my sanctuary. Then they began at the ancient men which were before the house.”
It is only those who mourn that shall be comforted by Yeshua. It means we understand the work Yeshua is doing now in the Heavenly sanctuary and are abreast with the times we are living in.
And it is in mourning that we become meek….and the scripture says….
 This is a prophecy of the sealing of the redeemed of the Lord, 144,000 as observed in Revelation 7.