John Gill Commentary Isaiah 34:8

"For Jehovah hath a day of vengeance, a year of recompense for the cause of Zion." — Isaiah 34:8 (ASV)

For [it is] the day of the Lord's vengeance
The time which he has appointed to take vengeance on antichrist, his 1260 days, or years; being up, in which he is to reign; these being expired, the time is come for the Lord to avenge the blood of his saints; see (Revelation 18:20) (19:2) : [and] the year of recompences for the controversy of Zion ;
the church of God, which has been for many ages abused and injured by the antichristian powers, for which the Lord will have a controversy with them; he will appear in favour of his people, and plead the cause of Zion, and recompense their enemies for all the injuries they have done them; then they that have led into captivity shall go into captivity, and they that have killed with the sword shall be killed with it, (Revelation 13:10).

This will be a time of double recompence; and therefore perhaps the word is used in the plural number; it will be the time of rewarding antichrist as he has rewarded others; and it will be the time of the dead, that they shall be judged, and rewards given to God's servants the prophets, (Revelation 18:6) (11:18) .

The Targum is, 'the year of recompence, to take vengeance of judgment for the injury of Zion.'