"Those who choose to make excuses and continue in sin and conformity to the world will be left to their idols. There will be a day when they will not beg to be excused, when not one will wish to be excused. When Christ shall come in His glory and the glory of His Father, with all the heavenly angels surrounding Him, escorting Him on His way with voices of triumph, while strains of the most enchanting music fall upon the ear, all will then be interested; there will not be one indifferent spectator. Speculations will not then engross the soul. The miser's piles of gold, which have feasted his eyes, are no more attractive. The palaces which the proud men of earth have erected, and which have been their idols, are turned from with loathing and disgust. No one pleads his lands, his oxen, his wife that he has just married, as a reason why he should be excused from sharing the glory that bursts upon his astonished vision. All want a share, but know that it is not for them. 2T 41.1"
"And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory."
Both passages describe the second coming of Christ and the reaction of people to His glory. EGW expands on the specific regrets and realizations people will have at that time, aligning with the general mourning described in Matthew.