To proclaim the acceptable year of the LORD, and the day of vengeance of our God; to comfort all that mourn;
To proclaim the acceptable year of the Lord - the year of jubilee on which 'liberty was proclaimed to the captives' (Isaiah 61:1; 2 Corinthians 6:2).
And the day of vengeance of our God. The 'acceptable time of grace' is a "year;" the time of "vengeance" but a 'day' (so Isaiah 34:8; Isaiah 63:4; Malachi 4:1). Jesus (Luke 4:20-21) "closed the book" before this clause; for the interval from His first to His second coming is "the acceptable year:" "the day of vengeance" will not be until He comes again (2 Thessalonians 1:7-9).
Our God. The saints call Him "our God:" for He cometh to "avenge" them (Revelation 6:10; Revelation 19:2). Primarily Israel speaks. The Lord comes again as their God, to avenge them on their enemies (cf. Isaiah 61:3).
To comfort all that mourn. The "all" seems to include the spiritual Israelite mourners, as well as the literal, who are in Isaiah 61:3 called "them that mourn in Zion," and to whom Isaiah 57:18 refers.