John saw seven angels in heaven receive seven trumpets. The first three sounded their trumpets and various judgments occurred. Then it was time for the sounding of the fourth angel,

Then the fourth angel sounded: And a third of the sun was struck, a third of the moon, and a third of the stars, so that a third of them were darkened. A third of the day did not shine, and likewise the night (Revelation 8.12).

Something happened to a third of the heavens that their light failed. Normally, these celestial bodies are good sources of light. Consider this interpretation.

Men fight the church and resist the Gospel of Jesus Christ, so they are given this partial judgment, not to destroy them, but that men might repent. Ezekiel 32 had similar words for Pharaoh and his nation,

“When I put out your light,
I will cover the heavens, and make its stars dark;
I will cover the sun with a cloud,
And the moon shall not give her light.
All the bright lights of the heavens I will make dark over you,
And bring darkness upon your land,”
Says the Lord GOD.
(Ezekiel 32.7–8)

Thus, Revelation 8.12 was a warning from God to the enemies of the church, that God knew what they were doing to the church and a horrible destruction was coming if they did not repent.