Revelation 15
1 And I saw another sign in heaven, great and marvellous, seven angels having the seven last plagues; for in them is filled up the wrath of God.
2 And I saw as it were a sea of glass mingled with fire: and them that had gotten the victory over the beast, and over his image, and over his mark, and over the number of his name, stand on the sea of glass, having the harps of God.

Having the Harps of God means: they were prophesying. Who then were these people?

In the same book of Revelation, those who overcame the Accuser, the Devil, had the Victory "by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death." We can also read in Revelation 2:11: "be thou faithful unto death, and I will give thee a crown of life". and also: "He that overcometh shall not be hurt of the second death."

The people seen in this vision who got victory over the Beast were Martyrs. In Revelation 6:10 these souls had asked God: "How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?"

Now the same souls are prophesying that God will render a just Judgment and avenge their Blood over the Land of Israel:

Revelation 15:
4 Who shall not fear thee, O Lord, and glorify thy name? for thou only art holy: for all nations shall come and worship before thee; for thy judgments are made manifest.

After the above verse, Apostle John saw seven Angels came out of the Tabernacle of the Testimony in Heaven.

After the Exodus of the Children of Israel from Egypt, a Tabernacle was built according to the plan and details given by the Lord to Moses. It was divided into two parts, the Tabernacle of the Congregation and the Tabernacle of the Testimony.

In the Tabernacle of the Congregation, the Priests performed their daily Services. The Tabernacle of the Testimony was open only one day each year during the Day of Atonement.

Hebrews 9
3 And after the second veil, the tabernacle which is called the Holiest of all;
4 Which had the golden censer, and the ark of the covenant overlaid round about with gold, wherein was the golden pot that had manna, and Aaron's rod that budded, and the tables of the covenant;

On the Day of Atonement the Priest would make atonement for his own sins and also for the Sins of the Children of Israel. Part of the Day of Atonement Ritual is described in Leviticus:

Leviticus 16:
12 And he shall take a censer full of burning coals of fire from off the altar before the Lord, and his hands full of sweet incense beaten small, and bring it within the vail:
13 And he shall put the incense upon the fire before the Lord, that the cloud of the incense may cover the mercy seat that is upon the testimony, that he die not:

This is exactly the same scene that is described in Revelation 8. It was the day of Atonement because the Angel had the Golden Censer which is kept in the Tabernacle of the Testimony.

Revelation 8:
3 And another angel came and stood at the altar, having a golden censer; and there was given unto him much incense...

Revelation 8
4 And the smoke of the incense, which came with the prayers of the saints, ascended up before God out of the angel's hand.
5 And the angel took the censer, and filled it with fire of the altar, and cast it into the earth: and there were voices, and thunderings, and lightnings, and an earthquake.
6 And the seven angels which had the seven trumpets prepared themselves to sound.

On the Day of Atonement God rendered the Judgment that Jerusalem would be destroyed like Jericho if they do not repent: Seven Angels were given seven Trumpets.

The Children of Israel having not repented in spite of multiple Warnings from Heaven, it was also on the Day of Atonement that the final and irrevocable judgment from the throne of God was rendered: Jerusalem will be destroyed.

Revelation 16
5 And after that I looked, and, behold, the temple of THE TABERNACLE OF THE TESTIMONY IN HEAVEN WAS OPENED:
6 And the seven angels came out of the temple, having the seven plagues, clothed in pure and white linen, and having their breasts girded with golden girdles.
7 And one of the four beasts gave unto the seven angels seven golden vials full of the wrath of God, who liveth for ever and ever.
8 And the temple was filled with smoke from the glory of God, and from his power; and no man was able to enter into the temple, till the seven plagues of the seven angels were fulfilled.

Revelation 16
1 And I heard a great voice out of the temple saying to the seven angels, Go your ways, and pour out the vials of the wrath of God upon the earth.

The Day of Atonement in the Book of Revelation is a Day of Judgment from the Throne of God. After the Seventh Trumpet sounded, and again we can read:

Revelation 11:19
And the temple of God was opened in heaven, and there was seen in his temple the ark of his testament: and there were lightnings, and voices, and thunderings, and an earthquake, and great hail.